The reason you "demand" that they fix you car for free is that if putting in the wrong oil damaged your system in anyway why should you have the added expence of maybe replacing the compressor. I personally wouldn't let them get away with poor work at the very least they should refund your money if your not comfortable with having them do any other work. If they are not told about there mistakes they will just keep on screwing up other peoples cars. Hold them responsible. Don't just walk away. Your not the one who his wrong.