My cousin has a 66 fairlane GT which he purchased new. He now has over 250,000 miles on it with one engine rebuild. At 85,000 he drained the top loader and reloaded it with standard 90 w gear
oil again. I agree with Bruce Edwards on staying with what was recommended. He has never had the transmission apart, not even for syncronizer rings. I'd say that's pretty good.
Just my 2 cents. By the way, it's a big spline toploader behind a 390 FE.
Mark