Now, following the manual that came with the mower, find the air cleaner. It is usually the top of the engine. This filters the air going into the engine when it is running. You need to clean or replace the filter located inside the air cleaner. The manual will describe how to do this. If the air filter element is foam, wash it with dish detergent in warm water, wring it out, and let it dry. Once it is dry, put some motor oil on it, wring out the excess, and it is ready to be put back in the air cleaner. If the air filter element is paper, it can be cleaned by blowing it out with compressed air. Direct the air from the inside of the filter out. Then hold it up to a light. If you can see light through it, you can reuse it, but if you can't see light, replace it. This step is important because airborne dirt is the engine’s worst enemy.

Once the air cleaner has been cleaned, check your manual on where the oil drain plug is located. You will need a wrench, a shallow pan to drain oil into, a couple of rags to wipe up any spills, and a quart of good quality motor oil. Most lawn mower makers recommend SAE 30 oil, though 10W-30 oil may also be used in most mowers. When you have located the oil drain plug, usually on the bottom of the engine under the deck, carefully tip the mower so you can get at the plug. Remove it with your wrench by turning it counter clockwise. Have your pan ready to catch the oil but use caution, the oil may still be hot and can burn you. Next, let the mower down on its wheels with the pan underneath and give oil a few minutes to drain out. Then, rag in hand, tip the mower up again so you can wipe away any oil around the plug hole and reinsert the plug, tighten it snugly but do not over tighten it. Wipe up any spills and lower the mower back onto its wheels.

Now, find where the new oil goes in. If the mower has a dipstick, as most now do, pull it out and pour in a little less than half of the quart of oil. Wait a minute for the oil to run down into the engine and then check the level with the dipstick. If more oil is needed, add only very small amounts until the level on the dipstick indicates full. Do not overfill the engine with oil. That can cause damage. It is important to change the oil after every 25 hours of operation because most mowers do not have a filter to clean the oil. Dirty oil causes serious and often fatal internal damage.