You should change the oil in your lawn mower just like you change the oil in your car. Most manufacturers recommend changing the oil annually or after 50 hours whichever comes first. Changing oil in a lawn mower does not take much time, but it will keep your mower running at optimal performance. Most mowers use SAE 30 motor oil, which you can purchase at an auto parts dealer.
Start the mower and allow the engine to warm the oil for a few minutes. Shut the mower off after it has run for three minutes. The motor should be warm to the touch but not hot. Doing this warms the oil and removes more sediment when draining than draining cold oil.
Pull the spark plug wire boot off of the spark plug with your fingers. Bend the spark plug wire back so that the boot will not accidentally touch the spark plug. Clean around the oil fill area with a shop rag to remove debris and sediment.
Tilt the air filter or spark plug side of the mower up and place some newspaper and an oil drain pan under the mower. Locate the drain plug under the mower deck next to the shaft that connects the mower blade.
Remove the oil drain plug with a 3/8-inch socket wrench. Lower the mower over the drain pan and allow the oil to drain. Raise the mower after draining is complete and reattach the drain plug to the bottom of the mower. Tighten the drain plug with the socket wrench. Clean the excess oil drips from around the plug area with the shop rag.
Move the oil pan and newspaper out of the way and lower the mower. Unscrew the oil fill cap by turning the cap counterclockwise. If the cap is stuck, you can insert the shaft of a screwdriver between the two finger notches and turn the cap.
Place a funnel inside the oil filler neck, and pour SAE 30 motor oil into the oil fill hole. Most mowers will take 20 ounces of oil. The bottom of your oil filler may have a small dipstick or your indicator may be a notch inside the filler neck. Refer to the owner's manual for your correct measuring device.
Place oil filler cap back on to the filler neck and turn the cap clockwise to secure the cap. Place the spark plug wire boot back over the spark plug and push the boot in with your thumb until it locks into place.