Amsoil is good for 25,000 miles if you use there new style filter. The filter they have is better than Banks Baldwin which used to be the best. I never run mine past one year because of the possible contamination of the oil by moisture and other byproducts of combustion that no filter will remove no matter how good it is. I do use Black Stone Labs to test my drained oil for metal particles IE bearing wear ETC.

I treat all my gas powered motors to this same treatment and I have never lost a motor yet to loss of lubricant. I did blow up a few 460 Fords in my jet boat but that is under racing conditions and very high RPM's

There is a ton of information on the web about oil and many people have different opinions. I just stick with what works for me.

Another product I use is Tuf Oil. This is just not a slick oil like all the rest. You use it every time you change the oil. Back in 1983 the company sent me a sample to try in one of my companys work vans. It had the old style carb motor and after I put it in the RPM's jumped 750 RPM and I had to take a screw driver and turn the idle down.. That just goes to show you how slick it is. It will not cause a fuel injected car to do that because the computer controls the idle. This stuff works and you should see a little better milage to boot.
Here is the link for your reading pleasure.

http://www.tufoil.com/consumer/home.html