I'm of the opinion that you should just start the car and run it. I've had 1/4 tank of OLD gas in one of my 2gnt's before- I pondered if I should drain it, then decided it wasn't worth it. I live in SoCal so the fuel wasn't subjected to extremes of temperature. She ran just fine and I drove her to the gas station and refilled with 87.

I would NOT boost on old fuel though.

2gnt: '99 RS-T, killed by a toyota, pending rebuild...
Daily: Volt
Daily #2: '99 EVG ebike- STOLEN by PEDRO