I highly recommend having the oil level at the very top of the crosshatch area. The Rebel doesn't hold much oil, and if the level is already low, and it burns a little or leaks a little, it's bye bye engine. I don't believe the oil level affects the pressure. The crankcase is vented. It is a 360 degree twin, and unlike a 180 degree twin, crankcase volume is constantly changing while the engine runs. That's why having a properly operating crankcase vent is so important. But, when the engine is running, especially at highway speeds, most of the oil is being circulated through the engine, with very little in the sump.