Drew:
I am sure that Amsoil is good oil. That being said, IMO a 5 quart jug of Mobil 1 at WM and an OEM PF 47 filter are hard to beat. They run about $25 at the WM, and will last 5-7k at least.
GM says that the oil and filter should be changed at least once a year. At some point, I don't think we gain much by extending the oil interval beyond 12 months or whatever the OLM says.
I think one can try too hard to improve on something, and many times the return is minimal for our money/efforts. Kind of like rotating tires every 1000 miles.
Don
