i use just a k&n filter with every oil change on my 05 wrx with 94k on it. i use shell rotella t full synthetic i believe its 5w40 which is the only full synthetic. you can get it at advance auto or walmart even. its pretty cheap to it costs me around $30 for an oil change. if you check any of the forums the general concencus is that rotella t is the best for our cars.