It factors a variety of variables into an algorithm which determines when your oil should be changed....it doesn't directly measure oil contaminants, nor does it go strictly by mileage. For instance, lots of steady RPM highway speeds for lengthy periods might not trip the light until close to 8,000 miles whereas frequent short trips and high RPM driving might trip it at 2,000 miles.
If you're really concerned, I'd suggest you contact Blackstone Labs
for a free container and send an aliquot to them for a used oil analysis. IIRC it's $22 and it'll tell you w/ 100% certainty how your oil is holding up.