You know, I called Tower and no one really knew the answer. I had assumed it had castor because of the color and because it left some varnish when I spilled in on my engine head.

However, the Wildcat EXTRA quarts are less expensive, and for certain have some castor. It is very good fuel and I would get the Wildcat EXTRA over the Tower fuel.

If I had to guess, I'd say Tower fuel is repackaged O'Donnell fuel. Tower doesn't actually make anything, and they are affiliated with O.S. engines, which is giving away free O'Donnell fuel with purchases of their engines.

I just compared Tower fuel to O'Donnel fuel - quart bottles are same shape, fuel looks the same, for whatever that's worth.

O'Donnell doesn't mention castor or synthetic, either.