I recently took my car in for it's 15k oil change. i bought in my own oil. Synthetic oil by mobile one($30). usually when they fill in with 5 bottles of the stuff the oil level checker is always about a half inch over the mark), however this time it's was about a half inch under the mark. well my question is how do i know they used my oil or if maybe they forgot to get it out of the passenger seat while they were changing the oil, so instead of going back and adding the oil they decided to keep the oil instead. is there anyway to tell if the oil is synthetic or not? thanks oh yeah by the way my cousin works for honda in the service department and he says when bmw owners bring in there cars for service they never change the oil filter for them because they don't carry them, but they don't tell the bmw owners this. what do you guys think. are all mechanic crooks?