Using too much oil usually goes along with smoke coming out of the exhaust. If you are using too much oil, the oil has to go somewhere and it’s usually into the cylinder, but it could be leaking out of your engine.
If you regularly have to add oil, and your mower is not smoking a lot, you need to look and see if you have a leak. This will probably be at the bottom of the engine where the shaft that the blade attaches to, comes out. If the oil seal has gone here, you probably need a new/second-hand engine or a new mower.
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