Originally Posted by redav;
My question is: If you can measure those things the oil affects, and from those measurements detect that the oil is going bad, why can you not measure those things and detect that the oil is NOT going bad? Or, if the computer keeps a log of the measurements, and there is a steady decay with time, it's pretty easy to detect a jump back to the "new oil" measurements, at which time the computer will conclude the oil was changed & resets
These systems are not complicated. They look at the number of miles driven, how many times the car was started. Just those two parameters would tell whether you do hiway or around town driving. Keep track of how many times the car is started and also got up to full operating temperature. Those three bits would provide enough info to calculate when the oil should be changed.
As far as the reset default it has to be at least what they expect the maximum oil change interval could be. Why the chose 20,000 miles; could be they expect better oil in the future that will allow for longer drain intervals and it would be an easier software update if they didn't have to change that constant. I doubt it was sloppy programing.