Here is what GM says.
“If engine damage otherwise covered by warranty
was found to be unrelated to the engine lubricant,
then the consumer’s practice with regard to oil
change intervals would not be a relevant considera-
tion, and the warranty claim would be honored.”
Fuels and Lubricants Division,
General Motors Research Laboratories
“The New Vehicle Warranty would not be void
simply because an owner failed to use proper
engine oils or did not perform maintenance at the
prescribed intervals. Warranty applicability is
contingent upon the cause of failure.”
Service Policies and Procedures Department,
General Motors Corporation
From
http://www.amsoil.com/lit/g2488.pdf