Most mower engines are 2-strokes, they're designed to burn oil as lubrication basically, it's added to the gas. You don't need to change the oil. Unless it's a 4 stroke engine (unusual due to size/complexity), or you've got a transmission or some other motorized mechanical behavior.
I wouldnt say most are two stroke. In fact most mowers available for purchase at your local hardware store are going to be 4 stroke if not all of them. 2 stroke lawnmowers are a thing of the past. Everyone wants 4 stroke and self propelling now.
Hmmm, I guess that makes sense. It's been a while since I've bought a gas mower, roughly the late 90's/00's hah, been using electric/battery since then.