Jim:
Soon after I purchased my boat three years ago, I posted on the "Ask All Sailors" forum your very question about the "every time" check of oil. Got a range of answers, from "most definitely" to "why ... where's the oil going to go since your last shutdown and what do you expect to find everytime?" I would think that the answer is it depends on your comfort level. If you are using your boat frequently and the engine mostly for short periods just to get in and out of the harbor, then say a check once every week or so = once every several times out is enough, which is what I do. Not too many people check their car's oil every time its driven (if ever). If you only go to the boat say once a month, then a check everytime will give peace of mind and also will prompt a general look-see around the engine for general condition of hoses, coolant levels, a check for fluid in the catch area under the engine, etc.
Another area of widely varying opinion is the need for a new oil filter with every oil change. My Yanmar manual specifies an oil change every 100 hrs, but a filter change every 300 hrs. A lot of people will opine that a new oil filter with every oil change is a must ... nothing but the best practice for an expensive engine ... cheap insurance. But as I've done with cars since a teenager, I change the filter every other oil change. Not because of cost, but changing filters can be messy and depending on the car necessitates putting it on stands to get at the filter. With modern engines, I also think that quality of manufacturing and tight tolerances greatly limit the amount of carbon blow-by and metal shavings that might be flowing through the oil and caught in the filter. I've never had a mechanical engine breakdown in the 40 years I've owned cars and motorcycles. And I generally own each one for 10 years or more. And the many times I've taken the valve cover off for valve clearance adjustments, I've never seen any buildup of sludge. So I'm doing the filter every other time with my boat's Yanmar as well.
