Service has to watch out.
Technically the Magnusson-Moss Warranty Act says that unless a manufacturer provides replacements for free (ie. plugs, air filters, tires, etc.), they cannot void the warranty because one uses an aftermarket part. Now if the company can PROVE that the part DIRECTLY caused the failure, then they can say that. The good thing is that this Act puts the burden of proof on the manufacturer. So if the service guys can prove that the plug failed and cause dthe damage, then it's not covered. Of course if you disagree, you can get a third party arbitrator, and the company must abide by his/her decision.
A friend of mine took his honda into the dealer to get his windshield washer system fixed, and they told him that he'd have to pay because he had turbocharged it. Well, he knew a thing or two about the law (he's a lawyer), and told them that. They changed their mind. Obviously they had no way to show that the turbo had caused the windshield sprayer to fail. But these guys will try anything to get out of doing a repair. Pretty sad.