Notice I said ring and pinion, so it's good in the diff, not the tranny/front axle combo.
There you want to use GL5 spec. Redline 75W90NS, or MT90 GL4 spec that is better in cold weather. I actually run a 50/50 mix. The synchromesh is probably ok, I don't see any problems with some of it in the case, but 100% synchromesh concerns me, since it's not GL 5 rated, as far as I can tell.
Remember the transaxle houses the synchros and the front diff, so you need to address the needs of the synchros first, and that is no limited slip fluids. Also nothing to thick or you will not be able to shift in cooler weather.
Do you use the car on the track?
If so then simply replace fluids often, depending on the number of track events.
On the street, outside of drag racing all the time, it will be very very difficult to see the difference in all the lubes mentioned above. Besides drag racing will destroy way more than the tranny.