Fellow in marina with big Krogen repowered recently and was running 100% synthetic. A seal or gasket failed getting some water in the crankcase. If that happens with regular oil it turns milky and you know the problem. Synthetic does not turn milky and he missed the clue that it was a little over full and ran oil/water mix till the knocking started. I run synthetic in my vehicles after engine is well broken in and I might go synthetic if I repower the boat, but you can be sure I will notice if the level goes up.