Generally, warranty claims may not be denied just because you performed normal maintenance work yourself. You should keep accurate records though to substantiate your assertion you performed the maintenance. The factory warranty follows the vehicle, not the owner; new or used makes no difference.
Dealers may attempt to deny warranty claims for any number of items including but not limited to: tire size change, exhaust system modifications, engine computer mods, well you get the idea. However, the dealer must prove the mods caused the warranty claim, under provisions of the Moss-Magnuson Act. Then you get into a war of money vs big money, then it seems much cheaper to leave your vehicle stock than fight the mfgr and their money. Once any dealership voids your warranty, no other dealership will honor it after they call up your VIN on the computer.