At 4500 miles, my OLM was reading 70%; I changed the oil anyway, as the dealer told me to change the first time between 4K and 5K.
My 2009 Malibu had an OLM and I'd go 10 to12K miles and it would still read above 40%, so to be safe, I'd just change it. I was leary about taking it down to 20% as I think that would have been closer to 20K miles than 10K.
I understand basically how the OLM works... it counts engine revolutions, tracks operating temperatures, and monitors driving conditions (such as city vs. highway), but I don't see how it can adjust for certain things that cause deterioration of oil such as dusty roads, fuel or water in the oil, and so on.
I have two trip odometers. I use one to track oil changes and one to track gas mileage. While I will use the OLM as a guide, I do not trust it 100% for the reasons I have mentioned.