Oil analysis

[Kitt]

Here is where I get mine done: http://www.blackstone-labs.com

Test kits are free. You just pay when you send it in for testing.

[blue_goose]

I also use Blackstone, they are good to deal with.  I keep a stock of the sample bottles and have one of there pumps to take the oil sample with out dropping the oil.  Take it before you change and you may find that your oil is fine.
Jack

[TomC]

If you get your oil changes at Speedco, they have an oil analysis machine right there for instant results. From their analysis, I found my oil change on my truck to be at 12,000mi-easy to keep trac of-just change every multiple of 12,000mi on the odometer.
Most Detroit dealers will do analysis also. Most of us change oil too soon. The bottom line answer of when to change it is by oil analysis. Good Luck, TomC