got in the truck this mornin to leave for work, started up fine nothing strange. i let it warm up for a few mins. put it in gear an took off jus like i do everyday. well it start studderin like it does if i run out of fuel then it died an coasted to the stop sign. its got half a tank of fuel. sat a cranked on it, nothin doesnt even try to start. pushed it down the street back in front of the house an popped the hood an pulled the fuel filter an its full of oil. bowl is full to. i put a new filter an still nothin. oil level is fine. where do i go from here?? what causes oil to mix with fuel?