BMW engines have a so-called circular pressure lubrication system. In such a system, the oil pump draws engine oil from the oil sump through a strainer and delivers it to the oil filter. Through the middle axis of the filter, the purified oil enters the main oil line. When the oil filter is clogged, a special valve directs unfiltered oil bypassing the oil filter directly into the main oil line. Downstream of the oil filter, the oil pressure of approx. 4 bar is regulated by an oil pressure relief valve. At higher oil pressure, the valve opens and some of the oil flows back into the crankcase.
Channels for lubricating the crankshaft bearings depart from the main oil line. Through oblique holes in the crankshaft, oil is supplied to the connecting rod bearings, and from there it is injected into the cylinder bores and onto the pistons.
At the same time, engine oil flows through vertical pipes to the cylinder head to lubricate the camshaft bearings (shafts), hydraulic valve lifters and valve guides. The backflow block ensures that there is always enough oil in the cylinder head to lubricate these parts.
In addition, oil is also supplied to the chain (chains) timing drive from the tensioners, and for gasoline engines - also to the oil pump drive chains.