Yes.  The additives in the GL-5 are corrossive of the brass bushings and
synchros in the transaxle, and according to redline, will cut synchro life in
half.  Use only GL-4 in the transaxle.  GL-4 provides plenty of protection.
GL-5 is engineered for the greater stresses of a differential, where corrossion
is not an issue.
steve

Michael Townsend wrote:

> Hi all,
>
> I could not seem to find any Red Line GL-4 synthetic manual transmission
> lube.  Therefore, I used Slick 50 synthetic, which is a GL-5.  I would never
> use a Slick 50 product in an engine, but I thought it would be OK in a
> transmission.  Now, after the fact, I am starting to wonder if using GL-5
> might be detrimental in an 091 transmission designed for GL-4.  Is this a
> big deal?
>
> Thanks,
> Michael Townsend
> townsend@ericsson.se