I use 10w30 in my stang for the engine...
I don't think the tranny takes oil, hehe. If it does then mine's in trouble. I've never heard of any 5spd taking oil.
The rear end uses grease, not oil. Dunno what kind.
As said above. Look into the 99+ pi swap. there is a difference, though, between the 99 and 01+ heads. The swap makes the car 100% different. Not only bigger valves and better ports but also an added point in compression. If you did this, too, you would have to replace the intake manifold and that would get rid of the coolant leak. Your looking at about 2g's price tag for the swap. You can find good deals in strange places, though. I got my heads/cams/valves for 350 off a wreaked 01.