So I sent a note to Honda to ask how long gas can stay in the tank, and whether it should be treated with fuel stabilizer if it's been in there a certain amount of time. Here is their unbelievably clueless response: Thank you for reaching out to us. We appreciate the opportunity to respond to your message. Unfortunately, we do not recommend modifications on the vehicle as it may void the vehicle's warranty parameters. If there were to be an issue with the vehicle and the Honda dealership determines it is due to the fuel stabilizer, then they have the right to void a warranty claim. In regards to your second inquiry, we do not have information pertaining to this matter. We apologize we were unable to further assist. For more information regarding your 2018 Plug-In Clarity, please visit http://ev.owners.honda.com/Clarity/ModelInfo/PlugInOwnersManual?pageMenu=PlugIn2018 Kind regards, Honda Automobile Customer Service So, in other words, add stabilizer at your own risk, and it's anyone's guess as to how long gas can be kept in the tank. Apparently there is no "exercise mode" or anything of the sort that runs on a periodic basis, because if there were I would imagine that would have been part of their response. Then again, they don't seem to know anything about the car, so your guess is as good as mine.