My dads wrx has 30k miles and we are going to change the transmission and diff fluid. Should we just use subaru
fluid an aftermarket fluid and if so what kind? Also is there anything else we should do at 30k? Thanx

There's a link in the FAQ at the top of this forum to the maintenance schedules for just about every Subaru. Use that and decide what you want/don't want to do.

I used some generic non-synthetic "Sta-Lube" 75-90 for the tranny and
Red line NS 75-90 for the Rear Diff.

Car shifts very clean with 35K miles on the odemeter and no 1st gear grind issues. Some people report grinding in 1st gear and other gears when switching to synthetic tranny fluid, some dont...I guess it depends on the car/driver. Lots of threads in here about 30K maintenence