I would stick with 15k. It’s a good round number and there would be a cushion for those intervals when conditions are not the best like winter for example. On the other hand, if it took the better part of a year to rack up that 15k, then the M1 AP oil over 20k would be an option.
I stopped using Blackstone they aren't as highly calibrated as the lab I'm using now. I didn't believe it so what I did was take 4 samples of the same oil. I mailed them to Blackstone and Testoil 2 weeks apart under different vehicles listed.
I got the exact same numbers back on Test Oil and Blackstone were off my a few PPM.
I made sure to stir the oil good and slowly fill all 4 containers at the same time with a little going into each at a time.
As an aside, my father worked for Mobil in the 70s. His company car was used for some Mobil 1testing. They had him leave the M1 in the car for about 70,000 miles. He would replace the oil filter periodically and mail it in for testing.
My wife uses the car for real estate in the exurbs of DC (warrenton, va). She puts 18,000 miles a year on it. Anywhere from short tripping to 20 mile highway rides to stop and go to dirt roads.
The AP would work in your case. Going slightly over one year shouldn’t be a concern. The big question would be the economics and I’m guessing that running EP over 15k will be cheaper than AP over 20k. That is where Mobil shot themself in the foot.