Yes, if you don't use Sta-Bil gas treatment. I believe gas formulations these days only stay "fresh" for a couple of months. Probably best to only fill about a quarter tank and refill as it gets used by "normal" ICE operation for more power, heating, etc. Can also use HEV mode once in a while to make sure ICE gets exercised. And never a good thing to leave an ICE engine just sitting w/non-use. You bought a PHEV, not an EV. It's designed to use the ICE on occasion!