What oil do you guys recommend for an 03 wrx?

full or semi synthetic? 5w/40 or 10/40? I've looked around and different places sell both as recommended for the car.

I'm also going to change the air filter, worth going for a cossie filter or stick with a standard one?

 

10w/40 seems to be the general viscocity most people on here use - though mine is 10w/50. Also if go fully synthetic instead of semi. Oilman is the man who seems to know his stuff and he he might just pop up in here with some advice.

Air filters don't really add much IMO maybe 2/3 bhp? Personally I'd go for a green filter