Then and Now
Over the years, gear oil anti-wear additives have evolved, going from lead to phosphorus, and then to active sulfur. However, it was discovered that the active sulfur caused corrosion of soft metals, such as brass, that are used in differentials and transmissions. They replaced active sulfur with a deactivated or buffered sulfur that was not corrosive to brass, copper, etc. Although this solved one issue, a different one emerged. They found that as the sacrificial coating became stronger on one or more surfaces of brass or other soft metals, rather than just peeling off, it took a few microns of brass with it.
A traditional GL-4 gear oil, regardless of viscosity, has about half the level of sulfur/phosphorus additive that you’ll find in GL-5 gear oil. As a result, the bond layer is not as strong. It peels off without taking a layer of brass with it. It should be noted that the use of a GL-5 product in a transmission requiring GL-4 will cause the synchronizers to wear. Eventually, they’ll no longer contact the other half of the cone. Consequently, they’ll end up bottoming out before stopping the opposing gear.
Difference Between GL5 and GL4 Gear Oil?
Another key point is that while GL-5 oils claim they meet API GL-4 requirements for gear oils, which is true, it doesn’t mean that GL-5 oils are satisfactory for synchromesh or synchronized transmissions. Why? While they do meet the gear oil specifications, they don’t meet the transmission oil specifications. Moreover, API GL-4 and GL-5 categories do not have anything to do with transmission synchronizers. Therefore, problems arise when you try to use the same product in the transmission that you use in the differential (regardless of what the oil company Tech Support reps tell you).
If a manufacturer tells you that their GL-5 product covers GL-4, they are correct as far as EP protection. However, that’s only part of the answer. Furthermore, a manufacturer that claims their sulfur/phosphorus additive will not corrode the yellow metals is technically correct. But if there is enough sulfur/phosphorous additive to meet GL-5 protection, it will slowly eat away your brass synchronizers.
So, what is the best gear oil for your classic car? Make sure you buy a GL-4 oil designed for the soft metal components found inside your classic car drivetrain. This includes both your gears and your brass synchronizer rings.