synthetic alternative to 15-40 marine

I have a Malibu tow boat with 340 HP 350. Indmar specs. Pennzoil 15-40 marine only but states Synthetic oil that meets our requirement of 15W40and API rating of SJ/CJ4 is acceptable to use after 100 hours or operation.(Still with 50 hour change requirement) I wouldn't worry too much about this accept I had a propeller rep tell me I could gain 150 RPM at WOT with a good synthetic. I pull a barefooter that takes the full rated engine rpm at WOT and am looking to prop up a little. Read some of the other posts and am concerned about auto synthetics in my boat engine. I would generally be looking to Mobil 1 which we use at the Chev dealership in all HP applications. what do you think?
 
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6,991
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Everett, Washington
Fellow Washingtonian Amsoil makes a Marine 15w-40 gas/diesel oil, this oil has military spec rust protection. They have Heavy duty 5w-30 that is gas/diesel rated, as well as a straight 30wt. The 5w-30 would give you good protections with less windage losses. In fact a straight 30wt syn would do the same. Whether or not you want to eek out the last bit of rpm with a 30wt would be up to you though. I assume this engine has a roller cam? If it did not it would not be critical on zinc/phosphorus that some of the oils have. Type of cam would sway my choice on the oil to use. If it is a roller cam you would have more choices..... Mobile 1 should have something that should work. Delvac?
 
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18,449
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East of IGO
Cept all the commercial marine fleets use HDEO oils in their automotive /high speed diesels so much for the amsoil corrosion claims.
 
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3,558
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SE Pa
M1 15w-50 or Valvo 20w-50 VR1 Syn. Avoid 5w anything unless you can verify its performance under dilution. But me thinks your propman is making excuses.
 

PT1

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near the mistake
Find a new prop guy. The oil won't give you rpm. Remember: your prop is a gear in the water that has a gear ratio related to your driveline.
 
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Chicago, IL
I believe your propman is saying that an oil with reduced friction will increase your motor's output and give you more rpm's. synthetic oil is not any more slippery than other oils and will not give you more output - all other things being equal. now, if you want to use synthetic to ensure the best protection for your expensive marine engine.... thats another story - although one that may stir up some debate. my Volvo Penta specifies synthetic....
 
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6,991
Location
Everett, Washington
He is talking a 150 RPM, not 400RPM, I could see that being possible. But I would want to see it before I said that it was or was not fact. Going from a dino to syn and or a 30wt syn I could see it.....
 
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5,069
Location
Saratoga, NY
If you reduce the coefficient of friction and/or oil viscosity, you might see a slight increase in horsepower. There will be some increase but it might be so small it will go unnoticed. However, I'm not sure switching from a "conventional oil" to a "synthetic oil" will automatically give you that increase. There are "synthetics" of average quality and some mineral oils over excellent quality. So, it depends, depends, depends ... I switch between the two seasonally. I like synthetics for cold weather and conventional oils are fine the rest of the year. This is true for basically every application I can think of.
 
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3,023
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USA-Michigan
I have a customer with the same engine in their 2001 ski boat, Indmar 5.7L that calls for the same spec's. They are using the AMSOIL 10W-40 marine oil (WCF) and a EAO25 oil filter.
 
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3,029
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Florida
I am running 10w30 Mobil1 in my 40 Mercury. There is a noticeable difference in the RPM's, quietness, and smoothness through the power range. I have probably 10 hours on the current interval.
 
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3,558
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SE Pa
Outboards are very different from larger displacement I/Os and inboards. You can get away with a lighter oil in the former and perhaps play more with synthetics. Not so much for the latter.
 
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