I have the stock US rear Diff on my '02 WRX, but in order to swap in a 6mt, I installed a ring & pinion from a '93 Legacy Turbo.

Anyone have any thoughts on the best gear oil for this setup? I'm leaning towards 'the thicker, the better'.

Pat, I think I would stay close to the factory-recommended weight. My thinking is that heavier weight gear lube consumes fractionally more power from the engine from the frictional losses. I think an exception to this might be if you were competing in some sort of high-speed endurance event or rally.

I would probably lean towards a high-quality synthetic gear lube. I don't have a recommendation but a number of local guys who go to local road course track days swear by Royal Purple over Red Line. A good synthetic should be able to protect both the tranny and differential gears from shock loads of a high HP/torque engine without resorting to a heavier viscosity.

Just my 2¢. :)

--
0==WW==0
"…axles of evil…" - george w. bush

You're $.02 in my pocket.;)

I think what you say makes sense. I guess I was just concerned because I've read that putting a 3.90 r&p to a 3.545 rear diff just smells like trouble. I had it in my mind that it was only temporary until I could cull somemore funds together to buy an appropriate rear end.

Someone says that it would be my spider gears to go after maybe 2 or 3 hard launches. Couple that with the slight clunking when I start off in 1st gear normally, that pretty much ended my drag racing career; at least temporarily. I haven't launched it hard, yet, even during daily driving. I do have a trackday tomorrow, but don't plan on 'launching' there, neither.

Thanks a bunch.