Should a person run only Mercuiser motor oil in a Mercuiser engine ??

YES! Any variance will void the warranty, cause the engine to sieze, and cause your wife and daughter to get knocked up by the poolman.

Do a search, probably dozens of threads on the subject. If its an older Merc pushrod engine, an oil with more zinc may be preferable.

Royal purple HD. you'll have to find a dealer though. Can't buy it at the parts store