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jackson · 06-04-2020, 10:14 AM

I believe GL4 is hypoid gear oil. My 49/50/51 Ford Shop Manual says this:

"Use SAE 80 gear lubricant for all transmission and overdrive assemblies. SAE 90 gear lubricant can be used for hard driving in hot climates. NEVER USE A HYPOID AXLE TYPE LUBRICANT".

The highlighted part is as printed in the manual.

I guess if the Sulphur/Phosphorus additives in todays GL4 is low enough to not cause damage to bronze or copper (syncro's) it would be OK to use. I don't work my Merc very hard...so will stick to non-hypoid GL1

06-04-2020, 10:14 AM	#15
jackson Member Join Date: Sep 2010 Location: Lincoln, Nebraska Posts: 54	Re: Flathead transmission I believe GL4 is hypoid gear oil. My 49/50/51 Ford Shop Manual says this: "Use SAE 80 gear lubricant for all transmission and overdrive assemblies. SAE 90 gear lubricant can be used for hard driving in hot climates. NEVER USE A HYPOID AXLE TYPE LUBRICANT". The highlighted part is as printed in the manual. I guess if the Sulphur/Phosphorus additives in todays GL4 is low enough to not cause damage to bronze or copper (syncro's) it would be OK to use. I don't work my Merc very hard...so will stick to non-hypoid GL1 __________________ 1950 Mercury mild custom. Original flathead, re-built, bored .040 over, Max-1 cam, Offy heads, Edelbrock super duel with Stromberg 48s. Last edited by jackson; 06-04-2020 at 10:22 AM.