Heat the oil can be

lubricating antifriction: between piston and cylinder, are present between the spindle and bearing a rapid relative sliding, to prevent excessive wear of parts, you need to build two  oil expeller  between the sliding surfaces. There is sufficient oil film thickness will be relatively spaced sliding surface of the part, so as to achieve the purpose of reducing wear.
 cooling down: Oil can be re-distributed heat back to the sump tank to the air to help cool the engine.
cleaning Clean: good oil to  oil equipment  carbides, sludge, wear metal particles circulating back through the sump, the oil flow through, wash the parts produced work surface dirt.
 leak-proof seal: Oil can be formed between the piston ring and the piston ring, to reduce gas leakage and to prevent outside contaminants to enter.