The Moss Magnuson act say's that the dealer must prove that the item(s) you added are the direct cause of whatever warranty issue he is trying not to pay for. Be it add on parts or lubricants. As Dolt pointed out it might come down to some type of court fight, but it also might make the dealer think twice if he knows you understand the law and have the M,M act in hand. I don't think it is a good practice to start with a sharp stick in the eye, as was stated above talk to the owner about what you're thinking about and go from there.
