I don't know if I have water in my tanks but I'd like to add something
(methanol?) to them to make sure I don't by adding something (what's the
best choice?) prior to the next filling. I have three steel 330 gallon
tanks of #2 fuel oil in my basement connected in parallel feeding a
furnace fired at 2 gph flow rate.

I've asked several local oil dealers and discovered that they don't add
anything, nor do they stock any additives.