The GM Oil Life monitor is an algorithm that takes the following aspects of engine operation into account:
Time
Miles
RPM
Coolant Temperature (Overheat)
Coolant Temperature over operational time (How long at normal operating temp)
Coolant temperature (Heat Soak after shutdown)
Oil Temperature
And probably a few more that I've left out. It is far more than a simple time/hour meter and GM has a patent on the algorithm. Upon reset with specified oil (per Manual) the OLM starts with a certain pre-programmed value. Each parameter subtracts percentage oil life depending on whether normal operating conditions are met or exceeded. Therefore short trips as well as any overheats can be accounted for without input from the driver. In MOST cases it is EXTREMELY accurate.
In my early years on BITOG, back when everybody was doing 5000 mi oil changes, I ran conventional 5W-30 in a 2014 Chevy pickup to 12,000 miles, running the OLM to 0%. I then sent an oil sample to Blackstone. The results came back as expected, with all additives still available (depleted of course) and some oil life left. No unusual wear metals were noted.
At that time I was one of the first members to adhere to the OLM. On a side note, the GM engineer that invented the system was female. The monitor does not check oil quality, it is based on a model of the specified oil and how it wears out based on lab testing. Using Synthetic oil in a vehicle NOT specified for it will not increase the oil life as recorded by the OLM.