You should put just enough oil in your lawnmower for it to register full on your mower dipstick. Make sure you give the oil time to settle before adding more. Overfilling the oil can cause the engine to catch fire. Clean oil will be clear on the dipstick so it is a good idea to wipe it clean once, and then check the level for accuracy. Most lawn mowers need to have oil added each mowing session because their oil container is so small.