GL-4 contra GL-5
The Difference between GL-4 and GL-5 Gear Oils - by Richard Widman
There is a lot of confusion about gear oils and the API classifications. In this paper I will try to
differentiate the two oils and clear up the mysteries that are flying all over the internet. It is extremely common, or normal, for all GL-5 oils to claim they cover the API GL-4 requirements for gear oils.
This is a true statement. Does that make them satisfactory for synchromesh or synchronized
transmissions? NO! They meet the GEAR OIL specifications, not transmission oil specifications.
The API GL-4 and GL-5 categories do not mention or have anything to do with transmission synchronizers.
The gear oils of a few decades ago had lead additives that were effective at wear reduction, but not very good for the environment. A long time ago they began to be replaced by gear oils with a phosphorous additive (in itself a decent anti-wear additive) with active sulfur to grip hold of the gears and create a very solid sacrificial layer of material that could be worn off, thereby protecting the gear surface.
Eventually it was discovered that the active sulfur was causing corrosion of brass and other soft metals used in differentials and transmissions.
Somewhere around 20 years ago a deactivated or buffered sulfur was developed that would react with the phosphorous to create the protective/sacrificial layer in the conditions created in the gear boxes (temperature and pressure) without being corrosive to the brass, copper, etc. This additive system is used in most gear oils today.
The problems arise when we try or need to use the same product in the transmission that we use in the differential. Many people have called oil companies and been told by the "Techs" that answer their questions that their oils have buffered sulfur and therefore are not corrosive to yellow metals, therefore their GL-5 oils can be used with brass components. While that answer is totally correct, it does not address the question asked: Can I use your GL-5 in my synchromesh transmission?
Lets take a look at the API GL-5 rating. It is a rating for EP (Extreme Pressure) protection. The higher the EP protection, the higher the GL category. In the mid 60's, GM developed the front wheel drive Oldsmobile Toronado that had a differential with a very high angle of contact for power transmission to the wheels so a higher category was developed (later to be called GL-6) to offer the protection needed.
This level of protection can still be claimed, but can no longer be tested since the Toronado rig used to
test it is no longer available. (Note: The 1966 and 1967 Toronados had sun gears between the axle
shafts instead of spyder gears and a very high offset, while suffering from the high temperatures of the
engine compartment and very high pressures.) This is why you will frequently see GL-6 listed as
"obsolete". The test is obsolete, not the car or its needs. Many other high performance cars continue to
spec this level of EP performance.
In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and
anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the
sacrificial coating of additives is pealed off or worn off. This is normal and acceptable in all steel gears.
But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger
than the base metal, and instead of just peeling off, it takes with it a few microns of brass that it is
A normal GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive
that would be in the GL-5 product, so the bond is not as strong, and therefore can be peeled off without
peeling a layer of brass (or less brass). This means that the GL-4 product provides a little less extreme
The Difference between GL-4 and GL-5 gear oils 1 of 10
pressure protection, so in the differential of a high powered car, it would not be the ideal product in the
differential. To understand this need we should be aware of the fact that the differential is where the
final torque is applied to the wheels (in most applications).
But in the transmission, we should consider two factors:
• Do to the fact that the differential applies the final torque, normally we do not need the full EP
protection in the transmission where less torque (about 30%) is applied.
• We need to be able to break the EP protection to stop the spinning of the gears long enough to
mesh them or synchronize them.
When we use a GL-5 product in a transmission that requires GL-4, we normally find 2 to 4 times as
much copper in the used oil as we would with a GL-4 product. Eventually the synchronizers wear to
the point that they no longer make contact with the other half of the cone, bottoming out before
stopping the opposing gear. (Refer to the picture below.)