I just got the speed network and i've seen nonstop advertising for ZMAX, it's endorsed by carol shelby... i dunno, does anyone use this?

Don't!! Most of these additives are snake oil and a waste of time

If you want the best oil and fuel additive IMHO (one of the few I would buy) try Lube Control products at:

http://www.lubecontrol.com/

See here for many favorable reviews

http://theoildrop.server101.com/forums/ubbthreads.php