An engine has all kinds of friction. It is inevitable. The crankshaft rotates in bearings, which have a clearance of 0.0012 of an inch. The connecting rods have similar clearance on the crankpins. The pistons are sliding up and down in their cylinders.
Oil films prevent all of these parts sliding and slipping past one another from scraping each other into a mass of silvery fragments. For oil films to form in all the places they are needed, oil has to be circulated throughout the engine.
The sump located at the bottom of the engine is deep and narrow to create a space for the oil that prolonged wheelies, braking, accelerating, and turning will not at any time drive the oil away from its pickup.
The pickup takes oil from the sump and conducts it into a pump driven by a small chain from the gearbox input shaft. A pipe sends oil from the pump into the filter, particulates are removed, and the oil returns and enters a gallery, which runs underneath the crankshaft.
Drill holes come up from the gallery to each of the five main bearings to lubricate them. Another hole brings oil up to the cylinder head. The upper bearing holders for the camshafts have channels cut in them to conduct oil from one bearing to the other bearing.
Each camshaft is hollow. Each cam lobe has a drilled hole so that oil flowing down the middle of the camshaft is distributed to every lobe and every tappet. Other passages supply oil to the shafts in the gearbox and also the shift fork and its respective slots.