Past window = Overfilled - decreases crankcase air; increases pressure when a windmill effect enters; relieved out thru engine ventilation system and up to air box/ram chamber. Ride without redlining, no oil into ram chamber. More oil will hold the heat and not transfer faster into the metal, where the air is cooling off the outside, more oil is holding that heat from moving out of the engine.
At or somewhere in the middle of the window = Proper level in the upright position/both wheels on the ground. Has a required level setting as per owner's manual. Has a balanced air to pressure ratio where ventilation will not travel up to the ram. Level pressure is the balance or we blow out seals, etc. Pressure is like electricity and if the seal is easier to push than running up the breather, then it found the shortest path. Bike runs with enough oil to remove heat and dissipate that heat with said quart capacity.
No oil in window = Underfilled - Engine heats up with less oil holding that heat inside, the air outside is moving in a hot ambient so the water jackets take on more heat. No time for that oil to cool, so it is pumped back thru the parts. Spin up the engine, you add air between the oil, now air and oil tend to run up the pump, less pressure, on comes the oil light.
A few scenarios goes something like that.
Tormenting the motorcycling community one post at a time