"I can give specs from my 1988
"I can give specs from my 1988 manual, knowing Mercury may have updated info since this was published. I did not notice, if given, what year engine you have.
Mercury recommended Quicksilver (of course) 4-cycle marine engine oil. If not available, a good grade straight weight detergent automotive oil of proper viscosity with an API rating of SF.
SAE 20W for nominal temperatures below 32 degrees F.
SAE 30W for nominal temperatures from 32 to 50 degrees F.
SAE 40W for nominal temperatures above 50 degrees F.
If the straight weight oil is not available, a multi-viscosity 20W-40 may be used. A second but less preferable choice is 20W-50.
In any case, non-detergent oils are not recommended.
For what it is worth, I have always used 20W-40 oil, but likely to switch to 15W-40 Rotella since I use that in truck anyway and my current cars no longer call for 20W-40."