i run amsoil 5w40 european formula in my 1.8 turbo and use a k&n oil filter and i havent had an issue so i agree with the needing of the 5w40, as far as the 3 month thing i change my oil every 3000 miles and thats maybe every six months and my motor is clean doesnt overheat and i droped my pan the last oil change and i dont have much deposit, id be affraid of any kind of a motor flush because you could open a big can of worms (like leaks) if the last owner (if used) didnt consider their jetta their baby like you do so a big