Back in the day O.S. did recommend using minimum of 18% oil content for their engines. That was a long time ago though. I do seem to remember using a lower oil content with my O.S. CZ-Z's. The thing is that O.S. doesn't recommend this no longer and hasn't for some time now (probably over 20 year or so). With the addition of synthetics less of an oil percentage is needed. The idea of running 18% oil in a modern engine like the one mentioned by the op is just crazy.