I think I have 75W140 Royal Purple in my rear differential. Thicker oil at higher temperatures equal better protection...and lower fuel economy. This is why racing oils are usually 40+ wt and go as high as 80 wt, they want maximum protection at high rpm/stress and don't really care about fuel consumption. Nothing wrong with 75 or 80W90 in any case.
From what I have read, the Redline transmission oil is better than others because it is claimed that other synthetic oils are actually too slick for the syncros to work properly. RL claimes to be the "perfect" oil for things to work well. Never used it, but sounds reasonable.
RL isn't always the best. DSMs for example are very picky. Some work best with RL, some with GM Syncromesh (my favorite) etc.
