I was thinking about running amsoil but the price is ridiculous. At ten bucks a quart it better be magic coming out of that bottle. I've heard good things about m1. Everyone wants to tinker a little with their machines, and shops around where I live are blatant ripoffs. Currently running a 3.4 05 impala with 192k. It runs quite well for her age. Doesn't leak and only burns maybe a half quart or so over 4k conventional changes. It has been well taken care of, at least the engine has... There's a reason we call it the rust belt. No condolences needed, the Jets shall rise again one day. I hope.
Their Signature Series oils are built for long drain intervals so the additive packages and basestocks used to accomplish that make it expensive. That is why they have XL and OE line of oils to be more price competitive while still offering either extended intervals (XL) or factory intervals (OE) versus the top-notch stuff (Signature) which is the longest OCI.
If someone is changing the Amsoil Signature series at 10,000 miles or less it's not worth the cost per mile in most cases.
Also don't forget with a yearly membership of $20-$25 you get 25% off the prices you see when you first arrive at the site. So if you are buying oil regularly it would make sense. Plus orders of $100 or more (easy to do) are shipped for free.
Not trying to convince you, just explaining the "why".
For folks like me that do a lot of miles in a short time (600 miles per week or more) it makes sense to do extended OCI's and have to change it less often, thus the cost per mile works out to be less expensive than more frequent changes of an economy lubricant not built for extended drain intervals and I get the added benefit of not doing the change in the cold winters as often. ALWAYS a plus.