Bill, the vast majority, (did they ever make a 2 sroke?) of Briggs and Stratton engines are 4 strokes. Oil in the crankcase, none in the fuel.
In '74 I moved my family to a remote property in a forest and provided for reduced electical consumption with a 5K McCollough generator with a B & S engine, which I connverted to propane, having a 500 gallon tank.
It never failed to start on the first pull, (once I figured it out), never caused any visable erosion to the single sparkplug I used for several years and if the oil was contaminated, it wasn't visable either, after an initial slight darkening while rinsing the engines internals.
Propane tanks are heavy, I'd figure out how to nail it to the mower, rather than hump it around on my back.
CNG (compressed natural gas) is even cheaper than propane, if you can compress it yourself, from your domestic supply. It's BTU content is lower than propane but the BTU per $ is still substantially less than propane.
Natural gas is difficult to compress into a liquid, very high pressure required but enough to mow a medium lawn wouldn't require the concentration of liquid compression and a big lawn could be served by more than one gaseous tank full, it's not like you are leaving home.
Bob