There used to be recommendations to change oil for time as well as mileage. Has that changed?
The standard recommendation is every 3000 miles for non-synthetic, which is very very wrong (too frequent) and serves to enrich quick lube shops at the cost of the environment. Folks with newer cars should NOT follow this recommendation [1], but instead refer to their car owner's manual.
[1] https://www.scientificamerican.com/article/oil-change-truths...
Also, oil changes every 6 months was mainly used as an appropriate time frame in case someone forgot to watch their mileage, with an average of ~14k miles driven in a vehicle a year that averages an oil change every ~7k.
The mileage based change intervals are about two kinds of degredation, shear (loss of viscosity), and Ph (exhaustion of wear additives). Shear may be improved by synthetics but additive depletion is not, although it is uncommon to run oil long enough for that to become critical.