I use Amsoil now in both of my cars (88 Mazda and 91 Tempo). Amsoil's 10w30 is claimed to go 25,000 miles on a non-turbo car, 12,500 on a turbo car. I probably won't even drive the Tempo that far before it should be changed due to time.

I still believe synthetic is superior to regular oil. And with extended drain intervals...it's cheaper.