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What oil to use?

Yes it is adequate.

My opinion and what I use is mobile one 15-50.

I feel the full synthetic oils are much superior to convential oils.

I just purchased 5 quarts of mobile 15-50 at Wallmart for $24.00..............
 
Should be fine. The availability of that oil makes if fairly cheap. Unless you plan on running the engine in cold weather, you can forgo the multi-grade component and go with straight SAE 30, or SAE 40. Make sure the API service rating is at least API SL/SJ or API SM. See attached for more info.
...I expect, as I write this, there's a synthetic oil activist congering up a multifaceted, epic length argument over the benifits of synthetic oil. All of which is true and beyond reproach, and will make the engine last a lifetime. I have no use for an engine that outlives the boat. Changing the oil regularly with good old petrolium serves just as well and costs significantly less. Besides, BP can use the revenue.
 
Marine Oils. by Chris Longhurst

I sometimes get asked "why are marine engine oils so expensive and why can't I just use regular motor oil in my marine engine instead?". Well, the National Marine Manufacturers Association Oil Certification Committee (click here for more info) introduced a four-stroke engine oil test and standard called the 4T certification. This specification is meant to assist boaters and manufacturers in identifying four-stroke cycle engine oils that have been specially formulated to withstand the rigors of marine engine operation. The certification was prompted by the growing influence of four-stroke engines in the marine market and their unique lubrication demands. So the simple answer is that regular road-based engine oil products don't contain rust inhibitors and won't pass the 4T certification. Lakes, waterways and the sea is a lot more aggressive an environment for an engine to operate around than on land.
Note : the NMMA have long had a similar specification for 2-stroke oils destined for marine use, called the TC-W3® certification.


Read more: http://www.carbibles.com/engineoil_bible.html#ixzz0qNKpwJDM
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Interesting stuff. I knew there was an NMMA cert for oil, but I thought that was specific to outboards. Couldn't find anything that excluded inboard/sterndrive engines, but the introduction in the NMMA Product Cert manual indicates its original interest in outboard engines.
Looked at my Merc service manuals, and they(Merc), in lieu of their Quicksilver oil, recommend API certified oil. A little dated, they acctually call for API-SH, which was superceded by SJ and again SM. Quicksilver oil is on the NMMA cert list, but Merc uses it in both stern drives and 4-cycle outboards.
 
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