I add extra castor to Omega too. Why? Because Omega also only has 18% total oil, part is castor. That's not enough to make me feel comfortable when running (especially) 1/2a engines on.

Goirish, with your mix of engines, I'd tend to squeeze towards the 20% mark, not much higher. 4 strokes can be sensitive to excessive oil content. It impacts their tuning, or so I've read. Still saving for my first 4 stroke.....

J