From the sump reservoir under the crankshaft oil is drawn through a strainer into the pump

or ball, valve that opens when the pressure in the oil exceeds the spring force acting on the valve seat .

When the valve opens, a return drilling is uncovered and the excess oil flows through this to return to the sump. This control is needed because the pump would produce excessive pressure at high speeds. After leaving the pump, oil passes into a filter and then into a main oil gallery in the engine block or crankcase .

Drillings connect the gallery to the crankshaft bearing housings and when the engine is running, oil is forced under pressure between the rotating crank journals and the main bearings. The crankshaft is drilled so that the oil supply from the main bearings is also to the big-end bearing bases of the connecting rods.

The connecting rods are often drilled near the base so that a jet of oil sprays the cylinder walls and the underside of the pistons . In some cases the connecting rod may be drilled along its entire length so that oil from the big-end bearing is taken directly to the gudgeon pin (small end). The surplus then splashes out to cool the underside of the piston and cylinder.

The camshaft operates at half crankshaft speed, but it still needs good lubrication because of the high-pressure loads on the cams . It is usual to supply pressurized oil to the camshaft bearings and splash or spray oil on the cam lobes. On overhead camshaft engines, two systems are used. In the simplest system the rotating cam lobes dip into a trough of oil. Another method is to spray the cam lobes with oil. This is usually done by an oil pipe with small holes in it alongside the camshaft. The small holes in the side of the pipe aim a jet of oil at each rotating cam lobe. The surplus splashes over the valve assembly and then falls back into the sump.

On cars where a chain drives the cam, a small tapping from the main oil gallery sprays oil on the chain as it moves past, or the chain may simply dip in the sump oil.

Some specialized vehicles use an oil cooler . The oil cooler commonly used is an air radiator similar to an engine-cooling radiator, with tubes and fins to transfer heat from the oil to the passing air stream. This cooler is fitted next to the cooling-system radiator at the front of the vehicle. Pipes from the filter housing carry oil to and from the oil-cooler radiator.