when are you checking your oil levels? try checking it in the morning before driving it so it can all drip down into the pan. your oil capcacity is 5.1 L, and if you're not seeing any puddles under your car, i don't know what to tell you. maybe you are burning tons of oil and u just can't see it? if you are, looks like you need some new rings. time for a leakdown test maybe.

btw a synthetic oil change only costs you $35 if you do it yourself :P (it's not hard) (and $35 gets you 5 quarts of mobil 1 and a mobil 1 filter)