Does modifying my car or using an independent mechanic void the warranty?

Not automatically. Under the federal Magnuson-Moss Warranty Act, a manufacturer generally cannot void your warranty just because you used an independent shop or aftermarket parts for maintenance. However, if an aftermarket part or improper work causes a failure, the manufacturer can deny that specific repair claim. Keep receipts and records showing maintenance was done on schedule with the correct fluids and parts. The FTC has a consumer guide on auto warranties and routine maintenance.