I am not a purist on this, but consider:

1) hours (especially idle time)
2) think months, not just miles

All my regular vehicles get the oil changed every 6 months since I don't put more than about 6,000 miles/year on any one vehicle (I drive a few...)

I do put my used race car oil in my lawn tractor.

Acids, etc can build up, and antioxidents can lose effectiveness over time.