Yes, you *can* add some castor.

The problem is that For break-in you probably want to raise the castor level to at least 6%-10% which will mean that your total oil content will increase to 24-28% by volume. This would do for break-in but would be unnecessarily messy and rob you of power for general running.

One thing to consider however is that the 75 is an ABC with quite a bit of pinch and (like all ABCs) should always be run using fuel that has some castor in it.

Yes, some folks do run their ABC engines on fuel with straight synthetic oil -- but those engines won't last as long and are at far more risk of damage from a lean run than engines that have a little castor passing through them.

If I were you, I'd just go out and buy a jug of Omega.