hockeynut,
If your Volvo was slipping in overdrive, how could that be the gearbox (and by implication, the lubricant). I can't see how a gearbox could slip without some serious grinding and knashing of severely worn gears. Wasn't your slippage really the clutch instead?
And, to progress the manual transmission lubricant war, I proudly state that I went with Mobil 1 synthetic ATF. Still working smoothly in all gears, upshifts and downshifts. I do find that sometimes it works best to shift into first before reverse when standing still and intending to back up. About half the time the reverse gear will grind if I don't do "first" first. When I do "first" first it never grinds.
Ken300D