Scotty, I purchased a 2015 Audi A4 Quattro 2.0T Premium Plus S-line automatic B8.5 with 40,300 miles on it.  It was my friends car that was killed in a plane crash.  Due to Covid, the car sat for a year without being driven. It had a half tank of gas and a dead battery.  I replaced the battery and started it up.  It ran a little rough for a bit but then settled down.  I'm going to replace all the fluids and filters, but I'm concerned about the gas. In your opinion, would a gas additive like Chevron's Techron Concentrate Plus Complete Fuel System Cleaner help boost the fuel that was left in the tank, or should it be drained?

Thanks for all your great advice!