What kind of additive? The problem for me is the low evaporation temperature of the ethanol.
And there is the rust factor caused by ethanol.
It's really not to pricey if you buy the gallon bottle, I just got tired of dosing out a few oz. at a time.
For awhile I had one of those plastic lab type bottle you squeeze to dispense a bit at a time, but I left it on a pump somewhere.
So lately I've been buying a similar but cheaper version from Big Lot's because it comes in a 16 oz. easy to handle bottle (plus it's $2) which I just slurp a little in each tank before fill-up.