If you are interested in how long you can go between oil changes, here is a site which sends you a free test kit, and will charge $25 to do analysis of your oil once you are ready.

This way, when you change oil, you will have a sample for them, will know how long you have driven this oil and the results will tell how much more you could have driven on that oil. The info can help stretch your service intervals safely.

Hope this helps.

Web site: http://www.blackstone-labs.com/