Years ago I ran Merccruiser conventional oil. It worked well for me. Then on a whim I tried Mobil One full synthetic. I noticed my running temp go down by 5 or 6 degrees. No leaks in my 1989 454, 330hp. At the time I had over 700 hours on the engine. I ran Mobil One from then on and ran that engine up to 1700 hours. So, I stick with synthetic unless there is a good reason not to.