The oil level in the engine should be checked and topped off as necessary before every use. All engines will consume a certain amount of oil while running. Most Walk-Behind mowers hold very little oil (less than a quart.) Therefore it is important to monitor the level and check for leaks on a regular basis.
Plugged or otherwise damaged air filters are the most common cause of internal engine damage. The should be checked at least once per month (more when mowing in dry or dusty conditions.) A plugged air filter not only robs power from the engine, it can allow dust and debris to enter the cylinder etc. The restriction caused by a plugged air filter can also cause the engine to suck oil from the crank-case causing the engine to use oil and possibly run low during use.
Blades should also be inspected regularly. Damaged or extremely rounded cutting edges cause the engine to work harder and greatly decrease cut quality. Worn lift sections at the back of the blade will decrease discharge and mulching efficiency which will also affect cut quality and engine loads.
While inspecting the blades, the under-side of the deck should be checked and cleaned of debris. Lush damp grass, such as early spring growth, causes debris to build up rather rapidly under the deck. This is often the cause of cut quality complaints. It will also have an effect on the rate at which blade wear occurs.
Another area to watch is around the engine shroud and cooling fins. If these get too restricted or plugged with debris it will cause the engine to run hot possibly leading to premature wear and/or engine failure. It is a good idea to blow off the engine periodically. This is especially true in the fall or other dry periods when the grass clippings and leaves are lighter and tend to build up more.
To scrape out your mower deck, tip the mower on it's side with the air cleaner up towards the sky. Use a chisel scraper or something similar to scrape off the grass build up. Note: If you tip the mower on it's side with the air cleaner towards the ground, the oil from the crankcase will flow into the air filter, causing it to become plugged. If you run the engine with a plugged filter it can cause a low oil level and lock up the engine. Replace an oil soaked air filter immediately and recheck/add engine oil.