Turbochargers

The turbo in the bus runs on bush bearings.   The oil flow and pressure is heavily restricted and the bearings run on a froth.  Inside the core, it's meant to be quite frothy. That's why the drain pipe has to be above the oil level in the sump, but also a large diameter such that the froth can drain away.  The pressure is not high because 'you' don't want oil getting past the seals. (aka piston rings).

(....  that's what I've learnt/been told.)

oil getting past seals I can relate to. Coming out of Brisbane from the Rocklea truck show in the Volvo F86 one year all good but getting out of the 'burbs I said to my passenger I don't remember this bank being so steep. By the time we went past the race track she was getting slower and exhaust getting black. Down a few gears and the exhaust was like those Russian crude oil locos laying a black smoke screen. Pulled up and it was apparent that she was running on her own oil. Cool down, turbo spins freely, top up oil and good for the remaining 400k's through the mountains of New England Highway.