Warranties are driven by the language of the agreement. If you are not in compliance with that language, you will be out of luck when it comes to enforcement. You need an attorney who works with contract cases to review the terms of the warranty to give you the best guidance.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Most likely yes they can refuse to honor the warranty. Most warranties require that their dealerships do the oil changes. By doing them yourself or having another place do the oil changes, it likely voided the warranty.
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.
This will depend on your local laws, and what your warranty says. It's not an issue of what they can do but what is legal. I like Rick Wynkoop on these issues in CO. Give him a call and tell him I said hello.