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Quick silver 25w40 is what you should be using in that engine.
The use of mercury 25w 40w is about all that is reccommended or needed if no preference is stated........period.
No need to look at a almost 40 year old oil spec. Oil has come a long way since then.
If you really want to do it right simply use mobile 1 15w-50w full synthetic and be done with it once and for all.......
for years the older stuff had straight weight reccomendations. But I bet to keep it simple and the lack of a special need (unless you live near the nort/south poles) a multigrade is all that you need. (of course this is for a pleasure boat and not some high power high horsepower motor.)
Also when was the last time anyone can remember hearing that motor oil cause a failure of any type buy using any wieght not reccomended by the OEM?
Nuff said