Interesting point but take into account the oil level doesn't exceed the full length of glass. In this scenario, it would likely be better to focus on the engine below the oil level near the bottom of the sump; however as heat is being applied externally to the engine, I'd have reservations about validity of this reading. It would be best to compare results with a thermocouple submerged in the oil but then after getting such a thermometer - the questions becomes mute.
Doug