The Castrol is good, many of the E30 M3 guys running
LSD with 30/30 ramp angles and 75% lock
up use it.
I have run the Millers 75-140 fully syn in
my E30 M3 DTM Diff, but its very thick
that you can not use the diff pump when oil is cold.
The Motul 75-140 comp is very good but very
expensive.
Some swear by Royal Purple, both in their
Diffs/Boxes and Engines.
What put 'Oil' into perspective for me a while back
was that when talking to Tractive about what
oil we should use in their 6 speed Sequential G/box
they said '' Use Coma, and change it a little
bit more frequent''.
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