What weight, and brand would be best to use in the 2005 WRX manual transmission?

The 05 WRX transmission uses 75w90. The capacity is 3.7qts. As for brand you have Redline, Motul, Amsoil, Eneos, etc. Also from memory 3rd gear is like glass if horsepower is above a certain point. Subaru designed it that way to tell if the vehicle had been modified power wise under warranty if I remember correctly. I had a friend go through 2 transmissions on his where he lost 3rd gear and only had 1st, 2nd, 4th, 5th, R. So if you're not pushing high hp/boost, you should be okay.