I thought I would pip in here and let you know something I researched about Amsoil and other synthetics.

My sponsor has been using Amsoil for years now in his truck. Back of the bottle says 35,000 mile oil changes can be done, you just have to change the oil filter every 3000 miles or so. He left his Amsoil in for 40,000 miles and then took about 1 ounce out to get it tested by http://www.titanracinglab.com/ The results came back that his oil was still good for use.

I started using Amsoil in my Civic and notice better mpg, acceleration and smoother idle. And most companies will tell you to change your oil at 3000-5000 miles because heck they're making more money. Even Mobil 1 Trisynethic you don't have to change every 3000-5000 miles. Talking to many racers some say wait til 7000-10000 miles - just making sure you change the oil filter.