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Best Gear Oil for Your Classic Car:

What Gear Oil Do You Need

Estimated reading time 4:00

Knowing what gear oil is the best gear oil for your classic car is critical. Doing so ensures your drivetrain and investment are fully protected. Classic cars are a source of enjoyment for millions of consumers. Maintaining them requires paying close attention to the required engine and drivetrain lubricants.


Modern lubricants contain additives that could damage the softer bronze, brass, or copper metals inside your classic car drivetrain. As a result, choosing the proper oil is extremely important.


Regarding gear oils, the American Petroleum Institute (API) classifies them via a range from API GL-1 to API GL-5. For example, the higher the number, the more modern the oil and, consequently, the number of additives in that oil. Therefore, you must know which rating you need when selecting the best gear oil for your application.


Over the years, gear oil anti-wear additives have evolved, going from lead to phosphorus, and then to active sulfur. However, they discovered that the active sulfur caused corrosion of soft metals, such as brass, 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. The industry discovered 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.


Regardless of viscosity, traditional GL-4 gear oil has about half the sulfur/phosphorus additive level 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’s important to remember that using 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.

What Gear Oil Do I Use? GL5 or GL4?

So, what gear oil should you use? While GL-5 oils claim they meet API GL-4 requirements for gear oils, it doesn’t mean that GL-5 oils are satisfactory for synchromesh or synchronized transmissions. Why?


GL-5 oils meet the gear oil specifications but 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 a transmission that you use in the differential (regardless of what the oil company’s Tech Support reps tell you).


If a manufacturer tells you that their GL-5 product covers GL-4, they are correct regarding 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 inside your classic car drivetrain, including your gears and brass synchronizer rings.


The experts at VP Racing Fuels have formulated the best GL-4 Classic Gear Oil line for classic cars. VP designs it as a yellow metal safe gear oil, which includes synchronizers, thrust washers, and bushings.


In classic manual transmissions and transaxles, you typically find these parts and other components. Our Classic Gear Oil is great for traditional manual transmissions such as Rockwell, Muncie, Borg-Warner, Ford, and Saginaw. It’s also excellent to use with import manual transmissions/transaxles that call for an API GL-4 gear oil.


Your classic car is an investment. You want to use only the best gear oil for it –  one formulated for the soft metal components found in a classic car drivetrain. When in doubt, ask the experts at VP Racing Fuels.


If you have an application that calls for GL5, VP has a Hi-Performance line of premium GL5 gear oil.


VP also develops Classic Non-Synthetic Racing Motor Oils, including  SAE 10W-30, 20W-50 oil, and SAE 30. We blend them with high-quality conventional base stocks.


In fact, many engine builders prefer conventional oil because they feel it provides better film strength and wear protection compared to synthetic oils. VP formulates its Classic Racing Oil with high ZDDP (zinc/phosphorous) levels and multiple anti-wear chemistries.


It provides extended engine durability and protects susceptible flat tappet cams and roller cams.


VP uses a significant amount of a unique base oil cut, which clings to engine parts. This affords additional protection to the valve train area and other engine parts. Moreover, the unique base oil cut provides wettability to engine parts even when the engine is idle for long periods, eliminating dry starts.



Get in touch with a VP expert