I have read about cars that have a pressure pump not unlike for fuel, but for oil. Why and how do they work? If you pump more oil into the engine, how does it get back to a reserve tank to be pumped in to the engine again?
The oil pumps were designed to transfer oil from a rear mounted tank usually called a Polarine tank. If you study the ancient race cars pictures where everything is much more exposed than on a stock car, you may see a small diameter tank about maybe 6 to 8 inches or so, right beside the gas tank, that would be the Polarine tank.
A lot of race cars especially used them and the "mechanic" who rode alongside the driver would monitor the oil consumption and every so often give a few shots of the pressure pump to transfer oil from the tank to the engine to keep the oil levels up. If we could see some of these ancient racing beasts perform on the dirt tracks or on cross country races, with poor or no form of air filters, I would bet the engines would have been smoking a little , so some oil consumption would have been going on, hence need for oil tank.
It's to spray oil into the exhaust, to fog the goggles of the guy behind you.
my old bugatti had an oil transfer pump..however it had no way of monitoring oil level!! since the oil pickup was on the right side of the engine the mechanic would monitor the pressure guage and when it went down on left hand turns he would open a valve and pump from a 4 gallon tank under his seat directly into the crankcase...because of the high viscosity of the oil a pump with a long handle [like an old farm well pump] was used.
also some early race organizers limited the amount of oil used in a race so instead of pit stops to add oil , the entire amount was carried on the car and added without a pitstop.
It didn't go back to a reserve tank, it simply leaked out or got burned. The pump was there to keep adding oil as the engine consumed/leaked it.