right from the Sea Horse's mouth
read the last line. says it all.
$20k boat and $6 propulsion unit, don't get cheap on the oil.
http://www.mercurymarine.com/serviceandwarranty/mercruiserfaqs/fueloil.php
What type of oil should I use? Can I use synthetic oil?
Engine Oil
To help obtain optimum engine performance and to provide maximum protection, we strongly recommend the use of the following oils listed in the order of recommendation:
1. MerCruiser / Quicksilver Synthetic Blend, NMMA FC-W rated, 4 cycle oil.
2. MerCruiser / Quicksilver 25W40 NMMA FC-W rated oil.
3. A NMMA FC-W rated oil.
4. MerCruiser / Quicksilver 4-cycle 25W40, non FC-W rated oil.
5. A good grade straight weight detergent automotive oil per the operating chart below.
NOTE: The use of non-detergent oils, multi-viscosity oils (other than as specified), non-FC-W rated synthetic oils, low quality oils or oils that contain solid additives are specifically not recommended.
http://www.nmma.org/certification/programs/oils/registeredoil.asp?y=FC-W2008