cali, I didn't use those numbers for convenience. If I used it, which I plan to figure out how much it will cost to switch over, I would have no trouble at all with trusting it for 25,000 miles.

It's not a miracle oil. It's designed to last that long. Yes, other synthetics are made to last twice your normal oil change interval, and cost $4/qt. They'd love you to only go 4,000 miles on their oil, because then you buy more.

Amsoil's oil is designed for "extended drain" which just means it's meant to go longer between oil changes.

I picked 25,000 miles because that is how it is stated on their webpage. I cut that amount in half for my turbo engine, or for hard driving. Even at 12,500 miles, it costs less. Should I calculate how many miles it would last to cost the same amount? What more do you need to hear?

No you can't just run any synthetic for 25,000 miles. They have to be specially made for it. Amsoil is, many others aren't. NEO (www.neosyntheticoil.com) is another that is made for extended drain intervals.