Synthetic oil can last an extremely long time and most people change it when they could easily extend the oil change way longer. I have sent Mobil 1 synthetic oil samples out to Blackstone labs which tests your oil. The results on my wife's Subaru with Mobil 1 0w20 at 8k which was 2k over the recommend 6k change showed the oil was still perfectly fine. The lab said this oil would easily run to 10k. I think 10k is a good number to use with synthetic.
Those oil life estimators are just guessing.
A better name is oil life estimator. They do not monitor any direct physical or chemical property of oil; they only accumulate data from the vehicle’s computer and predict how your driving habits and operating conditions have affected the oil’s viscosity, total base number (a measure of remaining detergency), oxidation level and other factors.
Since the OLM can’t measure these key properties like a chemist in a lab would measure them, how can it know when the oil has, for example, only 10 percent life remaining? It can’t – it simply estimates oil life based on an algorithm.