Hello guys. First wanted to say that I own a 1995 Four Winns Sundowner, powered by a Volvo-Penta (slash OMC) 302 Fuel Injected Ford Engine. I purchased it from the original owner, and he had run Penzoil straight 30 oil in since he owned it. Once I purchased it I swapped over the 15w40 Amsoil Diesel/Marine oil for a couple seasons until they came out with their 10w30 Synthetic Marine Oil. I know that the engine calls for 30w oil. After looking through my Volvo manual, it says use 30w oil. Do not use synthetic oil. My question is why not? I have been running the Amsoil in it with great success. Last year I had a riser gasket go on one of the manifolds, so the engine was pretty full of water. I ran the engine quite a bit without knowing there was a problem until I went to change the oil and saw all the water in it. Fixed that, flushed it, back to the Amsoil. The engine ran for several hours towards the end of the season with a ton of water in it, and I never (knock on wood) had a mechanical problem. But back to the original question, people say don't use multiweights in the marine engines because of the unburnt fuel and water that builds in the oil. Yet Mercruiser says use 5w40 oil in their engines. And again, why would the recommend not using synthetic oils? Volvo currently offers synthetic marine oil, and I am wondering that if they were simply behind the ball with oil technology. The engine is 14years old, synthetics were not as common or popular then. Plus, who knows when the first edition of that engine manual was even written.