Motor oil specifically marked as lawnmower oil should be put in a lawnmower. It is important to regularly change a lawnmower's oil to maintain the performance of the engine.
Before changing the oil in a lawnmower, start the motor, and allow it to run for a minute. After turning off the engine, remove the spark plug to prevent the engine from accidentally starting. Drain the oil, and remove the oil filter. After refilling the oil, use a dipstick to ensure enough oil has been added. A lift may be necessary to change the oil in larger riding mowers or tractors.