IIRC Wildcat is 18% total oil -- I can't remember if it's all synthetic or a blend.
At any rate, if you're running an ABN or ABC engine, you should be able to get by if you bring the total oil content up to 22%. You can do this by adding one cup (8 ounces) of castor to the gallon. It gets pretty close if the stuff is 17% total oil. One cup should fit in the free space at the top of the jug, and it's easy to remember.
Folks say "don't use medical-grade castor!" I don't know why, and I've used it. But it's hugely expensive. If you're going to do this a lot it's probably worth it to buy a gallon or two of castor from Sig to have on hand -- you can top off a whole lot of gallons of cheap fuel with a gallon of castor.