6.4 ounces
Casual, "good as it needs to be" answer: close enough.
Strictly: Nope. That is, indeed, 5% of 1 gallon (128oz). But it's only 4.8% of 134.4 oz, which is what you have
after you add castor.
Doesn't make much difference if you're only increasing by 5%. But if you want to go up 20% it makes a marked difference (25.6oz of castor would bring a gallon of fuel up by only 16.7%).
If you're not afraid of calculators, and you want to increase the castor (or nitro) content of fuel by 'p' percent, then you need to add
v_a = v_s * p / (100% - p)
where v_s is the amount you started with and v_a is the amount to add.
So if you want to add 5% castor to a gallon, p = 5%, v_s = 128 oz, and v_a (by calculation) is
6.7oz.
Just chuck 8oz in there -- it's close enough, and it's a nice round number!