Someone mentioned testing. You may be surprised to find you are throwing away good oil. I believe car guys use from 2-3 times the amount of oil they have to.

A few years ago my wife's primary transportation was a 1993 Chevy Suburban 4wd 3//4 ton. I was changing the oil every 5000 miles, the car had about 100K miles on its 350. One day I stumbled on a few Titan Labs test kits at a KMart for cheap. So I got the brilliant idea to test the Sub's oil. When I went to change the oil I found that instead of 5K miles it had more like 12K miles! Calamity! About half of these miles were city driving, the other half were Interstate at 82mph.
The oil was petroleum based, probably 10W-30 that I had bought on clearance at a Checker Auto for a quarter a can.

A week later I got the test results back. The oil was within spec and I could have used it longer!

I think we are so eager to show that we love our cars that we run way too much oil through them just to make ourselves feel good.

R.