Running old gas is a dicey proposition. I believe most folks think it might make the motor run bad, maybe run rough, maybe die, but have not experienced the more expensive effects of running old gas. Of course it can gum up your carburetor or plug your injectors, but the big expense is when it reaches an age that it causes your intake valves to stick. This can, and does, cause pushrods to bend, valves to bend, and can seize the valves in the guides so hard that they have to be drilled out. I have personally seen this on multiple engines - gotten to the point that when I see stuck valves, I immediately ask about the gas. It is worth the effort to drain the old gas: replace with good gas and you'll avoid a whole host of potentially very expensive headaches.