Diff Oil Specificiation

PNP

PNP

Member
Just wondering if anyone can enlighten me as to the difference between GL4 and GL5 specification gear oil and is the later (80w/90 GL5) suitable Diff Oil for a P6. Looking at changing out in my 68 SC Auto given I have no idea when itwas last done.

Also is that's typically used in the stearing box also?
 
I don't specifically know the difference between GL4 and GL5, though I do know that GL5 is not recommended for drive systems which contain 'yellow' metals. Lots of the MG differentials have brass/phospor bronze thrust plates in the diff, and apparently some of the additves in GL5 eats them. I'd stick with GL4 unless someone who knows more chips in!
 
That does make some sense. From what I have read one of the primary differences is that GL4 has lead as an additive and GL5 has phosphorous/bronze.
 
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