Well folks, this is probably about the tenth thread on marine oil that I have participated in over the last several years. As I posted earlier, there are many different perspectives. In this case, we are talking about a marine inboard engine. I'll stick to my original post -a good HD multi-viscosity auto motor oil will typically work fine - just change the oil and filter IAW the maintenance schedule. An outboard generally works harder than an inboard in terms of rpm's, operating ranges, and stress. So in my Honda 225 I use Pennzoil 10W-30 with an API certification of SG or higher, as recommended by Honda. I also religiously change the oil at the recommended every 100 hours. For me, that means about three oil changes every year.
The more important issues include (1) making sure your engine gets up to a sustained operating temperatures at each usage and to keep the engine decarbonized as much as possible. That includes using fuels that are pre-treated with a decarbonizer like Valvtect Carbon Free, or adding a decarbonizer at regular intervals like SeaFoam or YamaLube Ring Free, or similar; (2) using clean fresh fuel; and (3) using an inline fuel-water seperator, which should be drained every 100 hours and changed every 400 hours. Also, thermostats should be checked on a regular basis. The easiest way to do that is to install temperature gauges which will allow you to constantly monitor operating temperatures. When t-stats fail, they most often fail in the open position. That will keep your engine running too cool, and will allow water condensation to mix with your oil, greatly depreciating its effectiveness.
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