Over time, gradual impurities in the oil build up in the oiling system and can starve the bar and chain of proper lubrication. Dirt, sawdust and woodchips can also enter the tank and clog the pickup, preventing oil from reaching the chain. After many years, the oil pump can fail, but most often this isn't the case and the problem is either in the bar, the oil hoses or inside the tank, which prevents oil from reaching the bar.
Remove the bar, chain and clutch cover by undoing the two bar nuts with the wrench. Separate the chain from the bar. With the tip of the knife, scrape the length of the bar's guide slots clean. Remove all material from both sides. Pour a little gasoline onto the rag and wipe the bar clean.
Inspect the oiler hole in the bar, which is located just above and below the slot it uses to sit on the two metal bar posts. Clean that with the tip of the knife. Wipe it off with the rag. Put the bar and chain back on to see if that solved your problem.
Remove the bar, chain and clutch cover again if your chain is still not lubricating. Unscrew the oiler plate, which sits right next to the bar posts, with the screwdriver. Start the chainsaw again and depress the throttle. A small dribble of oil should come from the oiler nipple.
Stop the saw and remove the oil cap if no oil was present from the nipple. Drain all of the oil from the tank. Pour a little clean gasoline into the tank. Put the cover back on and swish it around for 10 seconds. Open the cap and pour the gas out. Clean the inner tank with the brush and rag.
Insert the metal hook into the oil tank and pull the pickup and oil hose towards the tank opening. Grab the pickup and oil hose with the needle-nosed pliers and pull them into the tank opening. Pull the pickup body off the hose while keeping it steady with the pliers.
Replace the pickup body, strainer and connector. Reassemble them in reverse order. Fill the tank with oil and check whether that solved your problem.
Only certified professionals should service the oil pump as you can cause serious damage to the rest of the engine if it's not done properly.