I wasn't so much concerned about oil vapors igniting; the flash point of motor oil is typically around 400 degrees fahrenheit.

I was thinking more of any gasoline or other fumes that might evaporate from heating the oil.

When I would heat the oil for centrifuging, I'd wait until after it was centrifuged (and cool) to add a thinning agent (kero, gasoline, diesel, JP-4, etc).