I am still running a 1978 Skeeter bass boat with a Mercury XR-2 (150 jacked up to about a 175hp) I bought the boat and motor new, the only oil I have ever used is the Merc oil. The reason I stick with the Merc oil, my friend that owns the marina where I bough the boat advised me to use it. His idea of boat oil is like my idea of oil for a vehicle, stick with what you start with.
My friend did not, and still does not carry Mecury motors, I special ordered the boat/motor through his dealership with the Mecury motor. His motor of choice is the Evenrude. My old Mercury has been in most every major lake in five states, it has well over 300 hours of running and other that routine maintence has never given me a problem even though I only know one way to push her, hammer down and trim up. Most of our area lakes are almost dry so she doesn't hit the water as often as I would like today.
Good fishing...........