I know that many people prefer to hose their mowers down with a garden hose. I admit I have done it. Still doing that is risky. If water gets into the wiring air filter or engine you could really do some damage. Hand washing with damp rag is a drag but also your safest bet.
In the following guide we mainly focus on walk-behind mowers since the majority of lawns are served perfectly well by these machines. However if you have a lawn that is larger than half an acre you may want a riding mower. If you have a yard that is between a quarter and half an acre consider a self-propelled model. This basically means you do less pushing. Self-propelled mowers are especially helpful on hills. Also pay special attention to blade size. Look for a mower with a blade thats at least 20-inches long if you have a larger yard. Conversely if you have many narrow areas to mow a smaller deck the area within which the blade spins around is best.
Place a drip pan or other container on the side of the mower with the filler. Carefully tilt the mower so oil drains out and into the pan. Discard the spent oil into a proper disposal vessel. Now slowly pour fresh oil into the mower. For the specific type of oil you need check your instruction manual. Make sure to add only as much as your particular model requires. Remember to let the mower sit undisturbed for a few minutes so the oil settles properly inside the engine.