You can use a "marine" rated oil if it makes you feel better and by doing this you get more corrosion inhibitors in the formulation. Normal PCMOS and many boutique oils seem to show zero effects from the moisture encountered in a marine environment however.

This is from ehow, about marine oil:

"Oil Types to Avoid It is not recommended that synthetic, non-detergent oil, oil of multiple viscosity, oil with additives or any low quality oil is used for MerCruiser products, according to Performance Product Technologies."

Read more : http://www.ehow.com/list_7564536_mer...oil-specs.html

What oils are non-detergent?? And why would multiple viscosity be an issue (the factory Merc oil is!)

From Mobil 1 website:
Is Mobil 1 a Detergent Oil? Answer:
All modern high quality engine oils are detergent oils. A non-detergent oil basically has little or no additive. This would equate to an API SA or API SB oils which are ancient history. An oil with little or no additive will certainly shorten the life of the engine or most other equipment in which it is used.

Mechanical wear- Ie rubbing of metal surfaces together which leads to erosion or material loss.

Chemical wear- a great example would be "corrosion" of the surfaces/metals inside an engine. However the chemical additives in a motor oil can also cause chemical wear. Prime example would be an oil with too much ZDDP. tribocorrosion encompasses both above forms of wear and is the proper terminology.

I use Blackstone labs to test my used engine oil. I was changing my oil every 25 hours, the lab tests indicated that it was too often. After a few samples, it was determined the 50 hours was a safe interval for MY boat. Mobil 1 20-50 V-Twin for 10 years now with no oil related issues.

the best advise I can give about doing oil analysis is to find a company that calibrates their testing machines regularly. One thing you can do to find out how well the oil analysis company that you are using is doing on their testing results is try this, take 2 samples of the same oil that is being sample -

Have your buddy send one of the oil samples in under his name and address, plus send your oil sample in under your information. You might be surprise at how the tests comes back for the 2 oil samples that are from the same source.

Again you might be surprise, 2 oil samples coming back with totally different results. Try it is all I can say. If your 2 oil samples from the same source comes back with the same testing results or very very close - BINGO you have a winner and use that oil analysis service.

I always used the 20/50 V-twin too, and also changed +,- every 25 hours. And like you, never any issues oil related.

Which oil to use can be like Ford vs chebby......

Fixed .

Why do you use the V-Twin oil? I know a lot of sport bikers use it, because it is safe for a wet clutch, unlike normal synthetic oil.

But curious why you use that on the boat?

Thanks!

It contains more zinc than regular automotive oil. Zinc seems to prolong the life of Hyd flat tappet cams.

Well I will most likely get hammered here about this but the bike oils are more shear stable but I was told not to use them outside of a wet clutch system. The source also sells bike oils as well and totally does not recommend them outside of that..