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Gear Lube Oil

I’ve heard varying information on GL4 versus GL5 gear lube oil n regards to use in older transmissions (not differentials). There’s a large number of people that recommend only using GL4 in the transmissions because GL4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive and the higher level of the additive in GL5 oils can deteriorate the soft metals used in synchromesh gears. What say ye? Any thoughts?

In some older transmissions, the GL5 is just too slippery for the synchronizers to work properly. They rely on a certain amount of friction or drag to work.

Use the gear oil recommended by the manufacturer and you won’t go wrong.

What does the carmaker recommend? Anything other than that is an experiment, and quite possibly an inconvenient or expensive mistake.

Keith, the difference between GL4 and Gl5 is the amount of sulfur/phosphorous additive. The additive is a gear coating that wears off in place of the metal of the gear. Early lube oils used leaded additives to coat, or protect, the gears. Environmental concerns have phased those out so the sulfur/phosphorous additive was developed. From what I’ve gathered, that additive is harder than the softer metal of the synchros so the additive can deteriorate the metal. GL5 having a higher concentration is said to be bad for older transmissions.

Shanonia, when you’re working with 50 or 60 year old components it’s not as easy to just check with the manufacturer.

I’m mostly curious if anyone has any specific experience they can share.

Don X, I see what you mean. I am sorry for my original response, which may have sounded flippant. My experience with maintaining transmissions doesn’t go back all that many decades!

Not flippant at all and no need to apologize.

Maybe this will help.

My Toyota Matrix manual says GL4 or GL5 are OK.
Lots of people on the forums say don’t use GL5 in Toyota transmissions, or any with brass synchros.
Some GL5 lubes claim the be “yellow metal” safe.
Since there are some great GL4 trans oils (like RedLine MT-90) out there
that’s what I use and I cover all bases.

The company that I buy most of the parts for my old 1974 TR6, goes out of their way to tell you to not use GL5 in the gearbox. Apparently it’s hell on bronze parts.

Thanks for the link Keith. That was fun looking at some of those old manuals. I never knew manuals existed before I got one for my 68 Dodge and that opened up a whole new world. Got by with a wing and a prayer before that I guess, and a brother in law.

My 98 Pathfinder’s manual said to use GL-4 ONLY. Had a section in the manual saying the higher sulfur in GL-5 was not good with some of the softer metals (Brass and Bronze) used in that generation Pathfinders.