Tell your "stealer" to put the "Warranty Voided" in Writing! And insist upon it! By law, no one but the manufacturer can void your warranty,and it has to be done in writing!
I get so disturbed when these guys try to do this. One time a customer of mine went to his Toyota stealer, and was told his warranty was voided because he used a synthetic oil in the engine. Well, I personally went to his stealer and spoke with the person as well as his boss and told them to "put it in writing", and they started to "back pedel" pretty fast!
Also I bought a new (in 96) Seadoo watercraft and dealer told me I had to use their oil in order to keep warranty. I called the 800 customer service number to verify, and the guy on the other end of the phone said "They can't tell you that" You can use any kind of oil you want as long as the oil meets our specs! Went back and told the seadoo dealer what I did, and they too "back pedeled" Been using what I want since, and no problems.
Wayne
amsoilman