I tried the oil directly in the cylinder trick many years ago for my snowblower. For some reason I didn't put the spark plug back in, and that winter/summer I think the cylinder rusted. Because the following winter when I started it up, it smoked. It never did that before. I think it also lost a couple of horse power, like down from a 7hp to a 5hp.Have not seen data on this... but chances of both your units dying is slim. I would not worry about it.
Long term storage... technically you're supposed to put a little oil in cylinder through the spark plug hole and pull the rope to coat the bore with oil to prevent rusting. I've never done this. Would probably consider it if storing for longer than a year or so. You stop pulling when you feel compression... this ensures the valves are closed, sealing the cylinder.
So guys, don't leave the spark plug out!