Mobil 1 75w 90 Synthetic, Valvoline 75w 90 synthetic and have had the best performance with shifting and lower tranny noise with Royal Purple (RP) Max Gear 75w 90. It is rated for use in the differential requiring a GL5 rating and the manual synchro tranny requiring a GL4 for low corrosivity for the synchros.
I just replaced the tranny lube this past weekend with a fresh RP charge from switching over 2 years ago. The amount of magnetic fudge on the drain plug was small and was the least I have seen form all the other fluids I used over the 80k miles I have had the car since new in 1992. In my case the RP solved a bearing noise when in 2nd and 4th gear in the tranny. I am sure there will be those that disagree but it works for me. BTW I have never had grinds when shifting.
