VW Baywindow Bus - Gear Oil Selection

by Richard Atwell
(c) Copyright 2005-2011


Oct 2012 Update: Richard Widman has produced a great article on the requirement to use GL-4. It makes the very important point that GL-5 is an oil spec rating, NOT a transaxle spec rating. This isn't obvious and should help to cool down gear oil debate. Thank you Richard!

Get ready to rumble! Gear oil is no exception to the endless online oil debates.

You are either in the "old-sk00l use what works" camp like folks who've relied on using single grade engine oils or the "it's a modern formula that's been improved" camp like the multi-grade folks. Whichever side you are on, you are probably looking for data to back up your position and it has brought you here. :-)

The choice between using GL-4 vs. GL-5 in the manual transmission is at the heart of the debate (if you have an automatic, go ahead and use GL-5 because it's compatible).

I'll state now that I recommend that you should use GL-4 in your VW Baywindow Bus. You might wonder why I've taken this position because I generally have modern viewpoints about available products and VW maintenance. It's simply because I've tried many gear oils and GL-4 works while the others have caused issues which I'll address below.

This is not a technical paper on gear oil chemistry and its effects on transmission components. The point of this article is to raise your awareness about GL-4 and help you locate the available brands.

What did VW say?

GL-4/MIL-L-2105 80w90 is what the owner's manual recommends. Since the gearbox requirements haven't changed from the day they were built, and since the differential uses a hypoid R&P gear design and given that GL-4 was made for that type of gearing, disregard the specs at your own risk. I'm not trying to scare anyone...it's just that VW had some justification for making these recommendations (otherwise they simply would have said "pour in gear oil").

If you are thinking about using a GL-5 gear oil understand that it was designed for the automatic final drive only and for newer cars. The VW transaxle is an atypical design relative to others and the oil has to lubricate both parts of the transmission (gearbox + differential).

Most other rear wheel drive vehicles for which GL-5 is intended, have a separate differential that houses its own lubricant. Modern front wheel drive vehicles with the transaxle style transmission tend to use a motor oil like lubricant which is also inappropriate for our VWs.

FWIW, simply saying GL-5 works ignores the fact the the transaxle design is unique.


In the older owner's manuals, VW used to specify SAE90 for mild weather, SAE80 for cold weather and ATF for really cold weather. VW printed up guidelines at the time of manufacture and then superseded them later on when they felt that the quality of multi-grade fluids had improved.

Note: VW's later 80W90 multi-grade recommendation superseded all previous advice just like it did for engine oils.

Deciding which guidelines to follow is largely a feel good decision but in general multi-weights are a better choice than single weights just as they are for engine oils today: sometimes single weight oils are too thick for the morning temperatures. (Good luck finding a single weight gear lube these days anyway unless you are either in the tractor business, order it over the web or have a buddy in the lube biz).

Read the label:

In order to choose a gear oil, you have to carefully read the back label. GL-4 is the recommended API rating (American Petroleum Institute) and SAE 80W90 is the recommended weight to use. Sometimes GL-4 oils are identified by these other classifications:

During your oil searches, you will discover variations of the military spec. Be arare that MIL-L-2105 is not the same as MIL-L-2105D and MIL-L-2105E, etc. If the label makes no mention of GL-4 or MIL-L-2105 it was not recommended by VW.

Here's how the API defines GL-4:

The designation API GL-4 denotes lubricants intended for axles with spiral bevel gears operating under moderate to severe conditions of speed and load or axles with hypoid (see note) gears operating under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications where MT-1 lubricants are unsuitable. The manufacturer's specific lubricant quality recommendations should be followed. Although this service designation is still used commercially to describe lubricants, some test equipment used for performance verification is no longer available. Procedures to define this performance are currently being reviewed for adoption by ASTM.

So they are saying stick with the manufacturer's recommendations and that the test equipment doesn't exist to verify GL-4 compatibility against newer lubricants. No wonder the labeling is so confusing: it may or may not apply. In light of that lack of information, go with the manufacturer's recommendation. Seems obvious doesn't it?


So why is the use of GL-5 so fiercely debated?

Oils are made from base stocks, refined and then mixed with additives to improve them. As a general rule, GL-5's incompatibility is often due to the extreme pressure (EP) wear additives. To give the gear oil its EP rating, it's traditionally been easiest to add a sulfer based friction modifier like MoS2 (molybdenum disulphide) but the broken down sulphur left in the oil (you can smell it) has in the past been responsible for corrosion and putting of the vintage copper alloy that was used to make the VW synchros.

GL-4 differs from GL-5 by the amount these additive chemicals are mixed into the oil (often 1/2 to much less in GL-4). Since the same additives are present in GL-4, they have the same effect on the synchros but far less (at a point in time that is beyond the expected service life of the gearbox or the oil itself).

To get the GL-4 rating, the oil has to pass the ASTM D-130 test. This test determines how reactive the sulfer is against a polished copper strip. During the test, the strip is also subjected to heating to simulate the running conditions in the gear box.

Food for thought: back when the problem first appeared the industry confirmed that at temperatures above 250F, the synchros indeed corroded. Does the oil in a VW gearbox attached to an aircooled motor get that hot given that it lacks an oil cooler?

Many people have put in a call to an oil manufacturer to ask them about the suitability of their GL-5 product for VW transaxles and received an inconclusive response. They may be told it's ok without any details to back it up. Likewise, people may have heard it's ok to use but the manufacturer won't confirm this because they haven't tested and proved it (cost and liability). The tech on the phone may not even know any better and since you are trying to match a modern oil formula to a 30 year old transmission, chances of getting an accurate confirmation is slim especially if his computer system comes up blank concerning your vehicle type.

To obtain GL-5 certification, the manufacturer does not have to pass the ASTM D-130 test. This is the primary technical reason why you should avoid GL-5 at face value. So, if you put in a call to a manufacturer about their GL-5, that test result is what you could verify. You could also ask for the D-140 test which is another corrosion test but I don't believe it superseds the other test which is sulphur specific. Will that info help you decide what to do? Probably not as it's just a single data point.

See the problem? GL-5 doesn't have to pass a test known to cause issues with older VW transmissions? Will it work? Maybe. Is it designed to work? Probably not. That issue is at the heart of the chemical compatibility and all the doom and gloom warnings: you are left wondering whether or not long term damage is occurring.

Some people will claim that using GL-5 in a GL-4 spec transmission doesn't apply when the GL-5 is synthetic. This depends on the oil and cannot be generalized. However, it should be noted that many GL-5 synthetic gear oils do not use sulfer in their EP additive package and can pass ASTM D-130 although the label won't include GL-4.

Some have said, that if there is indeed a problem, $50 worth of synchros will cost far less at rebuild time than $500 worth of other parts that otherwise wouldn't have worn because the EP features of GL-5 would have prevented wear. This doesn't explain how GL-4 run transmission have run 250K miles and not experienced wear on those parts.

To add fuel to the fire, some GL-5 formulas have been known to permanently coat the synchros in a VW manual transmission reducing its life by 50%. This is what the VW transmission rebuilders tell me (I don't have any details on this or more info to back up this claim).

What about MT-1?

In recent years a new gear lube rating has appeared: API GL-5/MT-1. All by itself, MT-1 is just as non-suitable as GL-5 can be but in the GL-5/MT-1 combination some manufacturers when asked are now stating that formula is ok to use in place of GL-4. It is hard to verify if these gear oils are suitable for the transmission. Certainly, they have not done as much research as VW has done but at the same time VW is no longer testing new transmission oils with their obsolete models.

All that is clear is that the oil has to pass ASTM D-130 to get the MT-1 rating. This will determine if their EP additive package is non-reactive with copper. Big deal right?

For that reason, sticking with GL-4 is simply the best course of action for long transmission life unless you can truly verify mileage and shifting performance with another owner.

Does it work?

The debate heats up over the chemical compatibility of GL-5 and variants. In my opinion, choosing a gear oil should be both a matter of chemical and mechanical compatibility.

Even if the manufacturer states the gear lube is "non-corrosive", its may be too slippery for the VW style synchros. This means that while the synchros won't corrode, the oil is not designed to grab the hub the way GL-4 does and the transmission will have shifting issues. This can be a problem for factory parts that have been run with and "worn by" another lubricant for tens of thousands of miles.

In hindsight that seems obvious but the heat of the debate which often turn into a personal attack of your choice of lubricant tends to cloud the issue.

If you find a GL-5 that works for you, that's all you need to know, right? But then we go back to the long term corrosion issues. D'Oh!

If GL-5 immediately causes poor shifting characteristics, then it doesn't really matter what the potential for corrosion is: replace it. If you are worried about the potential for premature corrosion, then you either have to have some proof that a particular GL-5 is safe to use or faith of the same, otherwise you'll only sleep at night using GL-4. :-)

One thing that proponents of GL-5 often cannot explain is why do all the known GL-4 lubricants work great but several GL-5's have issues? Simply because there is a difference in the formulation and you cannot make the general statement that GL-5 is ok to use.

For this reason, VW owners have been recommending to others for many years to stay away from GL-5: it is the safe bet.

Combo ratings:

Some of the gear oils available are both GL-4/GL-5 rated. I have no experience with these but given the few API ratings that exist, these oils might be a good choice for our transmission even with the differences in various types of differentials, gearboxes and so forth. Based on the fact that GL-3/GL-4 gears oils are compatible with VWs, these may be similarly compatible.

There is a reason why the GL ratings exist and differ. It is unlikely that a new rating will appear that encompasses GL-4. This is not how the ratings are designated. To date, they have only appeared in combinations with each other.

Until I can verify the compatibility of a particular brand or you hear overwhelmingly otherwise, I'd advise you to stick with straight GL-4 from the recommended sources.

Along with GL-5/MT-1, GL4+ is another new labeling convention that has appeared. It means the same thing as GL-4/GL-5 combined on the label. Similarly, GL-3+ means GL-3/GL-4.

GL-4 (last sighted with Bigfoot in the Ozarks):

So why is the choice of gear oil such an issue? GL-4 can be almost impossible to find locally because so few modern vehicles on the road require it. You may literally have to visit every parts store in your area to find some so be sure to call ahead first.

Because of this fact and by misinformed recommendation, many people just put in GL-5. Mechanics pouring in GL-5 are either ignorant, unable or maybe just too lazy to source GL-4 (hoping they won't see you again?).

If you are persistent, you can find GL-4 locally if you are persistent. Stalube 85w90 is probably the most popular weight/brand/rating available in the gallon jug and even it's hard to locate sometimes (Try NAPA and Parts Plus).

If you can't find any GL-4, leave the factory fill in the transmission until you do. There is no point replacing the gear oil prematurely on somebody's recommendation with a potentially incompatible choice. If you are low on fluid because of leaks or just oxidization, you'll have to speed up your search for some GL-4 or pay some shipping charges to get some.

I've read that the heat of the lubricant is an accelerating factor so if your bus has been in storage for a long time with GL-5 sitting in your transmission it shouldn't be cause for alarm. If you are not sure what's in your transmission and GL-4 is hard to source, you are probably better off leaving it there for now unless you are experiencing shifting problems.


I can personally recommend the brands below in yellow based on compatibility (USA based list only, sorry). If cost is a factor, get mineral based oils. If you consider how little gear oil costs over the life of the transmission (you can hope for 250K miles) then get synthetics.

One of the more commonly founds GL-4 gears oils is the Stalube branded GL-4. Even though Stalube is rated 85w90 instead of 80w90, it still produces smooth shifting in mild weather (always warm up your bus for 5 minutes before driving so the gear oil can warm up).

Be sure to check the weight when you buy: some Stalube bottles on the shelf are identical looking and are 80w140 which is suitable. Before giving up on your local search, call CRC and get phone numbers for local dealers/distributors. Ask for Stalube 85w90 GL-4 (#SL24239).


NAPA is one of the better auto parts chains and they tend to carry products the rest don't. Their house branded gear oil is labeled NAPA Premium Performance Gear Oil, 80W-85-90, GL-3/GL-4/GL-5 part# 75-210. It's made by Ashland (Valvoline) and comes in a quart bottle for about $3.29.


Carquest 80w90 GL03/GL-4/GL-5 in a quart bottle part #780-32. The label doesn't indicate who makes it or gives many details but the MSDS sheet indicates Valvoline (probably exactly the same stuff in the NAPA bottle).


Recently I came across Pennzoil Gearplus SAE 80w-90 GL-4.

I've always seen the GL-5 quart bottles at the local auto parts store but my inquires into GL-4 always came back negative. One day, I decided to contact SOPUS (Shell Oil Pennzoil US) in Houston, TX and see if they could locate a vendor in my zip code. They put me in touch with their local distribution warehouse and they sold me a case of #21176 direct for $30.57. That's a great deal for 12 1.qt bottles since I only need 4 and Stalube costs about $17/gallon. I was also able to obtain some contact information for companies they sell to which might be able to order it for me in future (the SOPUS warehouse primarily furnishes oil to auto dealerships).

Boutique Oils:

Some popular high end brand names sell GL-4 rated gear oil but most are not. Some are synthetic, some semi-synthetic, some not. These synthetics are recommended:

VW Dealer:

The VW dealer may also carry GL-4. Ask for "G 000 500 0" but don't expect a bargain. I've heard the price can be as high as $20/quart and that was before all the prices went up following Katrina.

GL-4's I have not tried:

The gear oils below appear compatible but I have not heard any feedback about them yet (positive or negative). Amsoil tends to rename their products a lot so be sure to ask them what AGT (their old GL-4 formula) is now called.

If you are lucky enough to find a reseller for Liqui Moly in the USA, they make a number of suitable gear oils. Use their 75w80 where it's really cold all the time.


Swepco 201 deserves special mention because it is one such GL-5/MT-1 gear oil that is used by the Porsche community and it's sometimes adopted by VW owners figuring what's good for Porsches is great for VWs. This is a mistake IMHO.

What most don't realize is that 201 does not produce successful results in all Porsche transmissions. Further, while Porsche and VW transmissions share a similar design, one is based on Getrag technology and the other Borg-Warner which can imply they have differently lubrication requirements.

Avoid using Swepco 201 with a VW factory transmission. Swepco makes great products but their oil isn't compatible with the VW 002 and 091 transmissions in my personal experience:

Think about the wear that will occur when using that gear oil especially during warmup and compare it to the cost of a transmission rebuild.

Fill-up Tips:

lube pump

You need to use a special 3/8" drive 17mm drain plug tool for the transmission. (ACC-C10-7165 at CIP1 for $4).

First, remove the plug in the filler hole in the side. This will create an air passageway to help drain the oil (the transmission isn't sealed but this helps to drain it faster). Remove the drain hole at the rear to drain the transmission. Be ready to catch about 1 gallon of fluid in a large container that is shallow enough to clear the transmission.

Be sure to check the magnet on the drain plug for filings and chunks of metal. Filings are normal, chunks are not (gear fragments). Take a photo, clean off the old filings and replace the drain plug.

Because of the location of the drain hole in the side, it's easiest to use a gear oil pump to fill the transmission. Pumps can be found for $5-7 and they screw into the top of the bottle. The pump pictured is from Stalube and it fits their bottle.

80w90 oils are very thick and slow to pour in cold weather. It may even break your plastic hand pump. The best recommendation is to put the sealed gallon jug in a bucket and fill with hot water (watch the level and keep it below the spout!) to warm it up before you pump it or pour it.

Extreme Cold:

How does 80W-90 compare to 75W-90 in extreme cold?

Pour-in Additives:

One question I get frequently is from people asking if there is an additive out there that will eliminate the gear noises they are hearing. Either crunches or grinding or whining. Unfortunately, in these cases, the gear wear is so severe that nothing in a bottle can be poured into the transmission to fix or mask these problems that I'm aware of.


So what can we take from all of this?

Here's a new debate:

What's more worthwhile? Figuring out which newer gear oils are suitable for the VW or sticking with what works while it's still available? I've tried to do the former and encountered difficulties. In that regard, I'd rather just drive to the vehicle knowing the GL-4 gear oil is compatible and cross that bridge later when I need to.

Do we really need to debate whether GL-5 works or not while GL-4 is still available? It almost seems utterly ridiculous doesn't it?.



03/12/07 - Moved from FAQ
03/14/07 - Added Liqui Moly info
03/17/07 - Added NAPA info
09/08/11 - Fixed broken photos, added translate button, updated footer
07/15/19 - Google update: new adsense code, removed defunt translate button
12/28/20 - Added ESSO cold weather video