Did some research and we certainly need to stay in GL4 category. I've been seeing some positive reviews on the Motorcraft XT-M5-QS.
Here are the better performing synthetic MTF I believe will work in the 6MT.
Redline MT90 75W-90
I did not list Royal Purple 75W-90 only because it's GL4/GL5 rated. I'm going to try out the Motorcraft and I'll update this thread with my opinion.