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In this thread I have read the downside or possible downside to ATF and Synthetics, so why chance it? No one has mentioned a downside to 80w90 except maybe the cold weather. I have always used gear oil but I cant say I have had any with original fluid and of course most if not all have been from down here.



Ok...you're asking for the downside, I will provide it. 80w90 is fine.....IF its a GL-4, not GL-5 lube, and 95% of the 80w90's available ARE GL-5's.

GL-5 has extreme pressure additives in it, highly desireable in a rear end (ring and pinion). The problem is these additives are not long term compatible with the brass synchros in our 833's.

That's the downside, but many people do use it.