How much castor oil should I add to a gallon of fuel for added protection?

I don't believe any of the current quality fuels need any caster.

We only use it for break-in

I know people running Tornado with 9% pure synthetic who add 1% castor to it. Be sure your castor will mix with the fuel, some oils will nox mix....
Take a glass and put some fuel in and a drop of the oil before you will ruin a complete gallon

I am using Tornado and I add 2% castor to it.