Engineer here for a major automotive company. An older colleague passed along this oil life rule of thumb before he retired. It's too good not to share. He had reviewed over his career probably thousands of sets of oil analysis data, and this RoT is based on that.
Oil life in miles= 50 gallons of fuel consumed per quart of oil capacity, times MPG.
-Or-
"oil pan capacity times 200 is the volume of fuel you can burn before changing oil." 5L oil pan would go 1000L of fuel burn before changing oil.
The idea is to calculate the volume of fuel you can consume in the oil service, then convert that to distance using your fuel efficiency. So if you calculate 1000L of fuel burn between changes and you get on average 8L/100km, you'd change every 12,500 km.
This rule gets away from unsophisticated and obsolete blanket statements like "every 3000 miles" or "every 5000 miles" and focuses on the primary cause oil degrades-- fuel combustion byproducts. Yet it's simple enough to use across vehicles and applications. It accounts of cold starts and short trips vs warm engine and hwy miles. It accounts for engine wear and power loss to some degree.
If it helps you feel better, you can collect oil samples and have the lab analysis done. Or you can get good-enough-for-most-of-us optimization with some very simple math. And if your vehicle has an oil life monitor, it's doing nearly the same thing but with electronic logging of throttle position and engine temperature and such. This rule of thumb will get you about the same place as an oil life monitor and can be used to sanity check it.
Finally, the 200 scaling factor (oil pan volume to fuel burn volume) can be fudged up or down if you think it is warranted. A Factor of 180 would be 10% more conservative, for example.
Caveat: this is not for race cars or other vehicles that sustain very high oil temperatures and have abnormal oxidation rates.
… meaning carbon from direct injection fuel bypass of ports etc
