I have been using this stuff for the last 3 years as recommended by the previous boat owner. I have used 9 mouth old petrol with no problems on the boat that I now use it in the fuel for the lawn mower and strimmer with no starting issues after being in the shed all winter. In the past I had to empty the tanks and put fresh fuel in to get the garden equipment going.
https://www.ebay.co.uk/itm/STA-BIL-Stabil-Marine-Fuel-Stabilizer-Boat-Petrol-Additive-Treatment-236ml-8oz/254309469807?hash=item3b36068e6f:g:TEYAAOSwm3ldOYD3