The bar and chain on a chainsaw needs a consistent stream of lubrication during operation. If the chain and bar aren't lubricated the tip and bar will start heating up. As they heat up further, they can damage the bar, sprocket, chain and even the clutch assembly.
Bar and chain oil is the only type of oil that can be used to lubricate the bar and chain. Nothing else should ever be used to lubricate the bar and chain. Most chainsaw manufacturers also manufacture their own brand of bar and chain oil. These oils are designed to withstand the higher temperatures and friction that occur when the chain and tip heat up. Also, chainsaws are generally used in warm or hot conditions, which further increases the engine and bar and chain temperatures.
When using the chainsaw in temperatures at or below freezing the bar oil will thicken in the cold temperatures. This thickening will increase the friction and the wear on your chain, bar and sprocket. To prevent this wearing, it is necessary to thin the bar oil slightly. Thin the bar oil by adding 1 to 10 per cent of #1 diesel fuel or kerosene. Start at a lower rate of thinning and gradually work up to 10 per cent, only if necessary.
No other kind of oil should ever be used in chainsaws. While many places may recommend using any type of oil, this can seriously damage the bar, chain, sprocket, clutch assembly and, worst of all, the oil pump. The oil needs to flow from the tank, into the pump and out onto the bar. Other types of oil, like automotive oil or two-stroke oil, will not be able to move through the oil system properly. This will cause the piston inside the oil pump to stick and stop pumping.
In a Pinch
However, some scenarios may present themselves when you are far away from any store and have only a few cuts left to make to finish the job. In emergency situations, automotive oil, two-stroke oil and other types of oil is far better on the engine than using no oil at all. Without lubrication the tip heats up rapidly and damage ensues very quickly. Using these other types of oil should only be considered a last measure. After using them, dump out any remaining oil and clean out the oil tank with a brush, rag and a little clean gasoline.