audi ?'s oil,perf.mods

hey guys, first off im new to the audi family. i have just purchased a 96 a4 quat. 2.8. love the car and options but not so much the wallet end.lol. anyway my ?'s are as follows "MOTOR OIL" now i have always ran 10/40 through all seasons Ohio can dish never any prob. but with this i just want to dbbl check with some ppl that know more to these cars than flyby car buffs and pricks with an opinion. what does this beast call for and or is considered correct winter n summer. secondly ive just done a KnN kit from summit racing. my next mod i am planning on doing is a exhaust system im def. partial to the magnaflows, but what size routing should i go with after the cats. and should i run straights after the cats to the muffler? 2.5 or 3in? the sooner the reply the better. any other performance mod suggestions would be great as well. thanks guys i look forward to reading your replys in the near future .