Re: Confused about which oil weight to use
DixieJ, a very good idea to have your engine oil analysed. Hopefully it will give you peace of mind. The fact that the engine seized AFTER you shut it down may have saved it. They do heat soak a bit after shutdown - that is one reason to idle them for five minutes after a hard run.
The Yanmar GM series are very robust and survive well even in salt water. With the two that I have owned over the years I have probably exceeded 3000hours between them. Neither had hour meters and I tend not to worry about hours anyway - more about having a calendar-based maintenance schedule e.g. oil and filters every six months, zincs at 12 months, impeller at 18 months etc.
On both boats, for less than 50 bucks I fitted a 'T' piece and shut-off valve in the raw water inlet line to which I attach a garden hose for an end-of-season freshwater flush (the boat stays afloat in salt water all year and we don't have a freezing problem here). If you do this, it is absolutely ESSENTIAL that you start the engine on salt water and then only put fresh water in at a fairly low pressure, then shut off the salt water intake and regulate the fresh flow carefully so you don't over-pressurize. You will need to look at the exhaust outflow to gauge this. Run it in gear at 1500 to 2000 RPM for a good 15 to 20 minutes to give it a good flush, keeping a constant eye on the exhaust outflow. You want it to get up to operating temperature. Then with the engine at idle, after a few minutes to allow it to cool down, shut off the fresh water first then shut the engine down, NOT the other way round (unless you want water in the cylinders!). By taking the hose off at the dockside tap first, by the time I get back to the engine it has drawn most of the water in the hose through and I shut it down immediately. You don't want to run the impeller dry.