On the warranty issue. Where does it say that it will void the warranty? The car manf. are carefully to not say that anything will void a warranty. They recommend. Further more, how would GM or a dealer know what oil you are using without doing a analysis and even then its iffy if it can be determined if an oil is API or the brand. They can give a best bet but most labs will be hesitant to offer a 100% conclusion without having a product for comparision.
Some will say that your receipts will or should state what oil you use etc. But that is not often the case. We had several vehicles at work, we always changed the oil every 5000 miles with no time limits. All the receipts ever said was; oil change, lube and filter. I don't recall ever seeing anything stating only API cetified oils were used. Sometimes the invoice would indicate what oil the sell but there was not statement what was installed, for all we know they put in anything that made them the most $$.
You sound like this is of concern, the API cert., so the XL7500 should be your only consideration. Under normal conditions, it procides a 7500 miles/6 months service. The lenght of service will depend to some degree on the quality of the air and oil filters you use. You have a choice of; 5W-20, 5W-30, 10W-30 and 10W-40 all which carry the API certification.
[ March 20, 2003, 09:12 AM: Message edited by: Mike ]