As far as old gas goes, empty your tank and carb bowls and just dump in in the lawnmower and start fresh every spring. It is still cheap enough that it's not worth messing with the problems that old gas causes to your carbs.
On the other hand I use Berryman's in my gas to help keep the carbon from building up in my cylinders. I replaced my head gasket ealier this year for the second time in a year and I was amazed at the amount of carbon build up in just a few hundred miles. I was also amazed at how Berryman's cut through and cleaned the carbon from the piston tops and head cylinder chambers and valves. Pretty much just dissolved it. That's why I use it. Doesn't seem to affect performance just don't over due it.