You could use a 10W30, but you might as well put in a 20W. After time that's what it will become.<br /><br />A regular dino passenger car 10W30 engine oil (PCEO) has viscosity improvers. Since a marine engine runs at such high rpm's at longer times than a car the VI's degrade and you are left with an oil that is out of grade -- something like a 20 grade.<br /><br />That is why the manufacturers state to run a straight grade 30 or 40 with a dino. Synthetic doesn't have the same issues so you can run with a multigrade. Merc's 25W40 is actually a blend of 25 grade and 40 grade oils so there are no VI's to degrade. You get the protection of a 40 grade and the cold startup of a 25. Don't know about the ratio.<br /><br />If you wanted to run a dino oil and weren't keen on the straight 30 or 40 grades, you could use a diesel oil (HDEO) such as Chevron's Delo 15W40. It won't thin out of grade like a PCEO will.
Here is Merc's "official" recommendation. It supercedes all previous owner's manuals.<br /><br /> http://www.mercruiser.com/fueloil_recommendation_faqs#1124 <br /><br />Too bad it doesn't reflect their new synthetic blend product
-- which doesn't even show up on their web site, but it has been advertised in national magazines.
