I would go for the 90 myself. It is the recommended weight. I have seen posts where a 50% mix if 90 and 140 was put in the rear axle. I think this was in cases where there was excessive noise or leaking.
I think the 90w would be the one to get. If you decide to run RP in the crankcase, it WILL show you where you have the least little oil leak. Ask me how I know. The oil is great stuff, the molecules are just so small that they will find any nook or cranny to hop out of. We run it in pumps and turbines at work that run round the clock for years at a time. The stuff is also made a little north of Houston too, so it helps the "home-boys" too. The film strength is pretty good too.