I'm trying to warm the oil, not the engine. I want the oil at operating temp for an extended period. I'll let the car cool down before changing the oil, though.
Run mode is one major possibility, as is month oil is changed. However, those two factors are aliased in my data. The only winter oil change was also the only time I drove mostly highway. However, the three summer changes show the full range of variation in fuel percentage and flash point, from 0.5%/400F to 3.0%/325F.
The data points to oil... 5W20 gave the 0.5% point while 0W20 averages 2.7% fuel in the oil. However, I'm about to change my third fill of 0W20 oil, right after a 300 mile trip. so we'll see... If fuel content is still high, the lighter weight oil is implicated independent of run mode. I'll be putting in a 5W20, likely Mobil 1 EP.
But I've retired, so miles are down and trips are shorter still, and that's a different driving mode altogether! Results from here will be slow to come, and not as consistent. Cie la vie!