It has been my experience that a fresh engine with new valves and guides will seize up in a short time sitting with old gas while a worn engine usually won't. I would guess there is an oil coating on a worn engines valve stems. A squeeze bottle of lacquer thinner with a "straw" nozzle will cut the varnish in the guides quicker than anything else I've found. Pulling the valve cover and dosing with thinner BEFORE cranking over can save headaches and dollars. After treating one can hear a snap when varnish dissolves, especially if the rockers are backed off.

Evan