Transmission Oil

[Ross F-1]

Originally Posted by JSeery

At lot of the GL-5 producers claim they are now synro compatible. I tend to not trust them and GL-4 is easy to come by. It is mild EP, which is what Ford specified and I don't care much for the smell of GL-5. I use Mercon myself (T5).

Here is a piece I copied off a different forum, explains things pretty clearly. Note that it applies only to Valvoline's High Performance lubes, they have others that may not qualify.

In that same thread, it was stated that the active sulphates in other GL-5's only corrode/discolor brass when oil temps are in the 250°+ range, not something I'd expect in an old Ford.