Thank you nemo, that's helpful.
I have two older cars, a 1999 Buick LeSabre and a 1992 Ford Ranger pickup, both with over 150k miles, and I think it's because I take care of them well. I consider oil changes to be cheap insurance.
I know that you can go longer between oil changes with the synthetic oils out now, like every 5k miles, but I prefer to use conventional oil in those vehicles, and just change it at 3k miles like the manuals say, or when it turns from from a nice honey color to dark brown.
My 2002 Maserati Coupe requires fully synthetic oil, but I'll still change it once a year even if I haven't put 5k miles on it since last change.
It's not worth it to do things the cheap way.
For further reading on how oil degrades:
http://www.upmpg.com/lubricationnews...tes/index.html
And how fuel treatments work:
http://www.jaylenosgarage.com/video/...tment/1234233/
http://www.jaylenosgarage.com/video/...t-gas/1315390/