Just did a trans/diff oil change...

[Wired 24/7]

Used some redline MT-90 GL-4 in the manual transmission and 75W-90 GL-5 in the diff.

What do you guys think of these products?

So far I'm impressed with how much smoother and less notchy it has made the transmission feel. I can't tell much difference with the differential yet.

Question: Why does the manual suggest GL-4 for the transmission but GL-5 for the diff? I have been hearing that GL-5 may cause corrosion or damage to brass synchros and therefore GL-4 is recommended.

 

[gtx510]

MT-90 has always shifted in the transmissions that I've used it in.

GL-5 oils are for hyploid (rear diff) gears, they contain Extreme Pressure (EP) additives that don't usually work well with the synchros in a transmission.
GL-4 doesn't have the EP additives and is better for a transmission.
I think GL-4 also works in diffs with bevel gears? or something like that.

Some gear oils will be rated as both GL-4 and GL-5.
You can use those in a transmission or hyploid diff.