For a gallon, I calculate 8 ounces.

I did this quickly, so I may be mistaken, but:
.15(128)+x=.2(128+x) Solving, I get 8 ounces.

In 128 ounces, 1 gallon, of fuel with 15% oil, there is .15(128) = 19.2 ounces of oil
Adding 8 to this gives 27.2 ounces, but we now have 128+8=136 ounces total fuel.
27.2/136 = .2 or 20% oil.

You'll need to do this in a larger container than a gallon fuel jug.

The formula, as I make it, would be:

Oil Added = (fuel amt) times (new% - old%) divided by (1 - new%)