I can see running regular over the shelf oil if cost is an issue. As an Amsoil dealer myself, I find it better to use the product as I can get it at a discount. Generally, I'll place a large order, mostly items for other people that pick it up locally and I work out the costs so that I get all my products at no charge.
I've always taken my '99 Taurus into Ford dealer for service, however, it was out of warranty when I began using Amsoil products in it. The dealer never said anything, and one of the service advisers has been buying it from me for nearly two years and runs it in his 07 mustang V6. Last I checked, if an engine failure does occur and the oil level was found to be within specification, the dealer won't test the oil unless you give them some suspicion to do so.
For anyone interested in more 'oil info' check out this site:
http://www.bobistheoilguy.com/