Yep, this. Even if it doesn't damage the engine the cooler temps of a water cooled outboard motor could lead to the oil not combusting completely and fouling your plugs and your local waterways. Use TC-W3 oil; any will do. If you're price conscious, I like Supertech. If you want the absolute best typically it will be a synthetic blend like Pennzoil Premium Plus. I don't know of any full synthetic oils that are actually rated TC-W3. (sidebar on the synthetics is that many claim to be fine but are really targeted at direct injection motors, not motors that mix the oil with the fuel either manually or automatically). A 1978 motor was built to run on less sophisticated oil than even a basic dino TC-W3 and so will appreciate whatever you give it as long as it passes the rating.
