Ok, so after I change the oil, my car runs like a dream, I get better gas mileage, etc.. for maybe 600 miles. Then the car runs alright for a while, by the time it gets towards maybe 2000-2500 miles, it absolutely runs like crap. I always change the oil every 3000 but this is a bit ridiculous. This last oil change, I emptied about 4.5 qts of black-ass oil out of my H22, (5.1 qt capacity).. so I know Im burning some oil as well. My point is.. what the fuck? Does the type of oil I use make a difference here? I use 5w-30 because Jersey winters get cold, but a guy at work told me I should be running 10w-40. Would that make any difference? What about synthetic? I do drive mostly highway miles... I need opinions!