With the mower still on its side grab your socket wrench and wood block. Place the block inside the blade well. This is to prevent the blades from turning while you loosen their bolts. Remove the blade mounting bolt/bolts. A typical mower will have one blade like the Craftsman 37700 I serviced for this guide (one mounting bolt). Some like my Honda HRR216VKA have two blades an upper and a lower (two mounting bolts). Each blade should have at least two cutting edges.
More than 2100 buyers on Home Depots site recommend the Toro Recycler 20333. The most helpful reviewer found the powerful engine did an excellent job of chopping through foot-high grass and the rear wheel drive gave him power while turning. He also mentioned that he could almost jog with it. Another buyer notes that his Toro still runs well after three years of questionable maintenance. He does note a couple negatives: It is a bit louder than the Honda and if you go to fast the mulch is not fine enough.
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.