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**Bob B.** · 02-07-2024, 05:47 PM

The issue with GL-5 is that many GL-5 products have sulphur in them. The sulphur is to protect hypoid ring and pinion gears from scuffing wear, just as motor oil back in the day had sulphur in it to protect flat tappet lifters and camshafts. Same kind of wear. Sulphur however is corrosive to brass and other non-ferrous metals commonly used in some manual transmission synchonizers. That's why most manual transmission specified GL-4 because GL-4 does not have sulpher in it.

02-07-2024, 05:47 PM	#22
Bob B. Senior Member Join Date: Aug 2005 Location: Los Angeles Posts: 1,309	Re: SM 465 gear oil help The issue with GL-5 is that many GL-5 products have sulphur in them. The sulphur is to protect hypoid ring and pinion gears from scuffing wear, just as motor oil back in the day had sulphur in it to protect flat tappet lifters and camshafts. Same kind of wear. Sulphur however is corrosive to brass and other non-ferrous metals commonly used in some manual transmission synchonizers. That's why most manual transmission specified GL-4 because GL-4 does not have sulpher in it. __________________ 1967 GMC CM-2500 Camper Cruiser, 351E V-6, NP 435 4 speed, Dana 60, and factory A/C. 2012 GMC K-3500 WT regular cab, 6.0L Vortec, 6L90. Last edited by Bob B.; 02-07-2024 at 05:58 PM.