Engine oil performs several different duties, all of which are essential to the efficiency of an engine.  Oil lubricates, cools (absorbs heat), and seals at various points throughout an engine.

Consumption of oil by an engine is due to the way the ring pack is functioning.  Consumption may occur between the cylinder wall and the ring face, if there is not a proper fit and seal between these surfaces.  Oil consumption may also occur if oil is passing between the rings and the ring grooves on the piston if a proper seal does not exist.  In the latter case, the rings actually act as a tiny pump as they move up and down slightly in the piston ring grooves.  In both of the aforementioned instances, proper engine temperature is very important because thermal expansion of the metals keeps the surfaces fitting together tightly.  Engine oil provides additional sealing, as well as lubrication, between all surfaces that are making contact as the piston moves.

Proper operating temperature is very important to both an engine and the oil which lubricates it.  Extremely high temperatures will cause the film thickness of an oil to be reduced, thus allowing metal-to-metal contact.  Lower than optimal operating temperatures in an engine does not allow the oil to flow properly into small crevices, such as ring grooves, where a tight seal is needed to prevent consumption.

Temperature extremes (high or low) are more difficult to deal with in air-cooled engines than in liquid-cooled engines.  This is due to the conductivity efficiency of air vs. liquid.  Air is not as effective as liquid, i.e. antifreeze, in maintaining a constant temperature while conducting heat.