Amsoil advertises 25000 mile interval change on their motor oil i used on my Z.
I changed under 4000 always but its good to know the viscosity break down and other key aspects of the motor oil can withstand abuse for that long.
You would however want to change your oil filter every 3-4K miles as that could cause some issues rocking that for 10-25k miles!
Amsoil is the first in synthetics as well, quality stuff i used it in my Engine, Transmission and differential on my 350Z.
Also use it in my daily driver Jaguar X type in the engine and manual trans, and transfer case.
Both cars it made the transmission feel sooooo much better, also in my Z switching from Mobil 1 to Amsoil i noticed less of a friction feel in the higher RPMs... My gas mileage went up a couple miles as well
