Four-cycle trimmers, unlike their two-cycle counterparts, need a regular supply of oil to keep the crankcase properly lubricated. This requires an oil change and the proper amount and type of oil. If the oil is not changed regularly, damage to the crankcase and piston may occur.
Most four-cycle engines require a SAE 30 weight that meets the American Petroleum Institute's service class of SF, SH or SG. Due to the high engine speeds, which cause enormous heat and friction, only use a high-quality brand name oil. Most trimmer manufacturers also manufacture their own brand of oil. While not an absolute requirement, this oil will give you factory standard results, as the trimmers have been tested using this oil.
Oiling the Crankcase
If your trimmer came with a small bottle of oil, it's best to keep this bottle to measure the exact amount of oil needed for the next time. Open the oil cap and tilt the engine so it's lying on its back and the oil spout is up in the air. Remove the oil fill plug and pour the entire bottle of oil into the hole. If you don't have a small bottle of oil, use a small funnel and pour the oil in slowly, periodically checking the level so it doesn't overflow.
Checking the Level
Check the level of oil every time before you trim. If the oil is too low, damage to the engine will occur quickly. Set the engine, fuel tank down, onto a bench or desk, so that the cutting shield extends beyond the desk and is pointing down. Make sure no grass or dirt can enter the oil hole; wipe the area with the rag if necessary. Remove the oil fill plug and look into the hole with a flashlight. The oil should reach to the innermost thread on the oil fill hole. If it's not reaching this level, carefully top the oil off until it reaches this point.
Change the oil after the first 10 hours of operation if your trimmer is new. After that, change the oil after every 25 to 30 hours. Always change the oil while the engine is still warm: it will flow out better and clean out more impurities. Tip the engine upside down over a pan or oil container. Remove the oil fill plug and let all the oil drain from the crankcase. Refill the oil to the required level after you have removed the old oil.