yep.... the recommended weight is 25w-40...... as per mercruiser.
although.... any good name quality oil is fine. 10-30 10-40 20-40... it's all good. just keep an eye on the level.
I run Wally World 30HD in my (OMC) 4.3...motor has been in the boat 20 years, has thousands of hours on it and never been apart...still running fine.
see sig pic taken this summer
Dan, MerCruiser has always recommended Automotive straight weight oil (SAE 30 or SAE 40) or their own multiweight oil (Quicksilver 25W-40). They have never recommended automotive multi weight oil. Lots of folks claim to use it w/o a problem, however I would never take that chance.