The engine is a magnum 61RFS; they call for 20% percent castor only for break in. I have downloaded the instructions for OS, Saito, Enya and YS and it amazes me that all of these manufacturers differ on break in oil content and procedure, who is right and who is wrong. I know that the castor/synthetic argument has 1000 different sides to it but I wanted to follow as closely as possible the recommendations of the manufacturer.