What makes the oil get hot in a mower.

Oh that. That's very normal, it happens on every engine eventually if you're not paying attention. You have a very keen eye. That's the oil trying its absolute best to keep the engine from exploding. I know, I know, it's very annoying, everybody's busy, busy, busy. That problem is called EMPTY.

- See, the oil gets splashed around and up into everything it can to save you hundreds and thousands of dollars. When it's left to sit, "cool down" as you say, the oil drains back down to rest from the war it's fighting bare handed. You need to fill the only sump to the proper level. Then, you can drag it around like a rotting slab of meat for hours and hours. Enjoy.