Chainsaws operate at high speeds, with engines revving as high as 3,500 RPM. The motor drives the saw's chain around a guide bar to cut the wood, which generates a considerable amount of friction between the chain and the bar. Chain saw bar oil is used to reduce that friction, helping to extend the life of the chain saw's engine.
Bar oil lubricates the joints of the chain and helps reduce friction between the chain guides and the groove in the guide bar. This lubrication allows the chain to rotate faster, with less effort required by the engine. It keeps the chain and bar cooler, helping to preserve the life of the chain as well.
Chain saws have a reservoir in the body of the saw for holding bar oil. The oil is applied to the chain either manually, with the push of a control on the handle, or on some saws, automatically distributed as the saw is used. Auto-oilers are designed to distribute all of the bar oil in about the same amount of time it takes for the engine to use a tankful of gasoline.
Types of Oil
Most chainsaw manufacturers recommend using speciality bar oil rather than other types of oil. Bar oil is typically thicker than motor oil to adhere to the chain (where thinner motor oil may be thrown off by the speed of the chain).
Oils to Avoid
Some woodsmen have tried alternatives to bar oil, with varying degrees of success. One type to avoid is used engine oil. Used motor oil typically has lost some viscosity through it's use in an engine, and often has microscopic flakes of metal that can cause damage to the bar and chain.
Some other alternatives to bar oil include new motor oil mixed with generous amounts of thicker lubricants, such as STP. While this used to be a popular alternative due to the cost differential, in recent years, the cost savings have shrunk to the point that specially designed bar oil may actually be less expensive than the alternatives.