I recently changed my transmission and diff oil to Redline but I put 75w90 in the transmission as well as the diff, instead of putting MT90 in the transmission. Is this ok? The guy at the shop told me it should be fine since they're both GL5 gear oil?? Is it worth the bother to change it back if so?

Shoot an email to Redline. They are real good about getting back to you.

cool will do, thanks!

I just did my diff oil with RL 75W-90 (apparently the tranny oil changeout idea caught on so well that the MT fill/drain crush washers are backordered all the way to the factory....)

There was a note on the diff oil botte warning against using it in a transmission. Something about the sulfates in the diff oil being bad for the synchros, as well as an issue of the diff oil being "too slippery" and possibly causing some sort of functional issue in a MT.

o..... ok guess I'll go change mine again ASAP... sigh... thanks!

Originally posted by bgreene
as well as an issue of the diff oil being "too slippery" and possibly causing some sort of functional issue in a MT.

The synchros need friction to work properly.

GL5 fluid has high performance additives for extreme pressure use between surfaces, these are USUALLY sulphur based, and can be very corrosive to the brass parts (synchronizers) in a manual box.

GL4 fluid specs are much less demanding, and the additives are not needed.

MT90 is a GL4, and ideal for your gearbox, but I would get the GL5 out, pronto. If your gearbox should fail in the future, the technicians can usually spot the black/blue synchros, and your warranty is 'out the window'.

The Redline oils are, in my opinion, the best you will find, but always GL5 in the diff, GL4 in the tranny.
.
.
.
doc