Performing routine scheduled maintenance to a lawn mower engine will keep it running efficiently, and will increase the life of the engine. Changing the oil on a regular basis is a major component to proper engine maintenance. New lawn mower engines should have the oil changed after the first five hours of operation. After that first oil change, you should schedule of oil changes for every 25 hours to 35 hours of use. Consult the engine manufacturer's specifications for the types of oil to use in any specific engine.
Pull the sparkplug rubber connector from the top of the sparkplug. The connector is a heavy black rubber hood that electrically connects the engine to the sparkplug. If the lawnmower has an electric start option, remove the key from the ignition. Pull the positive battery cable connector from the positive post of the battery. Empty the fuel tank of remaining gasoline if the engine does not have an oil drain plug--you will need to tip the mower on its side to drain the oil if the engine has no drain plug, and you don't want gas to spill in the process.
Locate the engine oil drain plug on the lawn mower engine. It will have a square neck on top of the plug. Use the crescent wrench and remove the plug by turning it in a counter clockwise direction. As soon as the plug is free of the hole, oil will begin to flow from the orifice. Use the plastic container to catch the used oil.
Some lawn mowers may not have a separate oil drain plug. In this case, remove the oil filler cap. Turn the mower to its side and allow the oil to drain out. Catch as much of the oil as you can in the plastic container.
Turn the lawn mower right side up. Replace the oil drain plug if applicable. Fill the oil reservoir in the lawn mower engine with fresh oil. Check the level indication on the oil dipstick as you fill the engine. Wipe the dipstick with a clean rag, and check the level again.
Replace the rubber hood connector over the sparkplug. Fill the fuel tank with fresh gasoline. Start the engine on the lawnmower. Allow the mower to run for about 30 seconds. Shut the mower off. Check the oil level. Adjust the level of oil as needed.