View single post by Esprit2
 Posted: 12-03-2017 05:24 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 344
Frank, I don't know what JH's spec is for the 5-speed, so I'm not disagreeing with you as much as asking for clarification.

Not all gear oils are created equal. Those that carry an API GL-4 rating are intended for synchromesh transmissions. It is synchro-friendly both in terms of frictional characteristics and chemical compatibility with the brass/ bronze bits that are usually present. IMHO, Redline MT-90 is one of the best GL-4 gear oils.

Gear oils that carry an API GL-5 rating are anti-friction, anti-wear formulations intended for the hypoid ring & pinion gears often used in a rear axle. Old school GL-5 oils typically used sulfur-based anti-wear additives that are incompatible with copper and it's alloys, brass & bronze.

Some synchro transmissions (usually transaxles that combine synchros and hypoid gears in one housing) do specify GL-5. In that case it's usually a compromise between reduced gear wear and increased synchro chemical erosion. Both wear and erosion take time to occur, so what was the engineer's priority for this application?

The JH 5-speed does specify a gear oil instead of a motor oil, that's a given. But is it the same as the rear axle gear oil, or a different 'transmission' gear oil? I don't know. I'm asking.

Some modern full-synthetic gear oils carry a dual GL-4/ GL-5 rating. They use superior base oils along with non-sulfur anti-wear additives to meet the needs of both ratings in one blend. That might be the better option.

Tim Engel

Last edited on 12-03-2017 05:26 pm by Esprit2