You know guys, both of you make good points but got to tell you that you need to understand it is an excellent idea to keep receipts, however it is a far better thing to make sure you know what condition the oil (and the engine for that matter) is in if you elect to follow the recommendation of oil manufacuturers about extending the oil change intervals when using synthetic oil.
Remember the onwers manuals are not made for any specific type of person and they usually generalization maintanance that is not always something that is done properly. Many of the things that are omited in my manual I had to find in the component manufacturer's manuals.
The manufacture of my engine (Cummins) recommends a 7500 mile oil change on this engine (ISC)using conventinal oil. Right now I am getting to change it at 25,000 miles since my last oil change. The last OA came back clear with one indicator a little high, big deal. I run Mobil one, no blending, not anything else - just the real thing. This engine has 636,000 miles on it and still runs great (except for the Cummins updated fuel pressure sensor kit I have to reinstall). Now talking to the Cummin regional rep, he told me that because of the OA, it matters to them more than receipts.
My advice is simple; establish the habit of an oil analysis and if you decide to use a synthetic oil, do not use a blend but a pure oil but research carefully which one.
Leo, didn't see what you posted, good advice.