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I put about 50-60 hours a year on my diesel these days since I did something really, really stupid...getting a job. As the boat is not hauled for the winter, being that I live in Florida, I change the oil every 12-15 months. Given that change frequency, should I be using synthetic oil rather than "conventional" motor oil?

Thank you in advance for your input.

Ron
s/v Lucia
 

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While I am quite convinced that synthetic oil has some superior properties, namely lifetime, high temp stability, and viscosity vs. temperature, I don't think you're likely to take advantage of them. But worst case is you spend a bit more than you could have done.

Shell makes Rotella in standard, semi, and full synthetic versions. I refuse to believe that the synthetic version will destroy your engine when the standard version won't.
 

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It used to be, not to use synthetic for two reasons. If you burn a lot of oil, it was expensive. But not now. The other reason is if the engine is shut down for long periods of time, like the winter, the oil is the worst fogging of oils and the insides of the engine corrode. I guess that's still true. Your OK.
 

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Yanmar, who I suspect make more marine diesels than anyone else, will void their warranty on new engines if their oil recommendations aren't followed. The problem isn't a lack of lubrication with synthetic oil, it's that synthetics in cool running marine diesels glaze the cylinder walls and build up a very hard deposit on the edge of the piston crown that will score the cylinder. This subject has been researched fully and the results are easily available. However, don't take my word for it, read your engine's manual or get in touch with the maker. Or, to find out for yourself, run your engine for about 30 hours with a full synthetic and then check your oil consumption.
 

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Synthetics have also been reported to eat seals and gaskets.

Tell ya what: do some homework. You do a search on this and any other boating forum, use the term "synthetic oil" with the " " marks to get the exact phrase, and you will be able to read for weeks. Really.

Dino oil works just fine, follow the engine manufacturer's recommendations, avoid synthetic oil in older engines 'cuz those engines were made before synthetics were invented.

That's the short answer. Up to you to find the longer one if you so choose.
 

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I can't live with myself changing oil that infrequently. Although, I suspect it's within manufacturer spec (typically 100hrs or 1yr). I just think dirty oil, sitting in a marine environment is bad.

I do the filter once per year, but change the oil more frequently. There are any number of ways to set yourself up for it to be a quick deed on one of your visits.
 

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Hey,

Personally I don't believe there is any ADVANTAGE of using synthetic oil in a marine diesel. I also don't believe there is anything wrong with it either.

Here is the manual for the Yanmar 4JH4-HTE-1:
Use an engine oil that meets or exceeds the following guidelines and classifications:
API Service Categories CD, CF, CF-4 and CI-4

SAE Viscosity:
10W30, 15W40. Engine oil 10W30 and 15W40 can be used throughout the year

Here are the specs for Shell Rotella T (not synthetic) 15W40:
API CJ-4, CI-4 PLUS, CI-4, CH-4, CF (and a whole bunch more).

So Shell Rotella T is clearly allowed by Yanmar

Here are the specs for Shell Rotella T6 (Full Synthetic):
API CJ-4, CI-4 PLUS, CI-4, CH-4, CG-4 CF-4, CF (and a whole bunch more).

So I don't see how Yanmar could void any warranty work on engines that used Rotella T6 full synthetic.

Again, I don't think that a marine diesel would benefit from synthetic oil, but I don't see how it would hurt.

Barry

Yanmar, who I suspect make more marine diesels than anyone else, will void their warranty on new engines if their oil recommendations aren't followed. The problem isn't a lack of lubrication with synthetic oil, it's that synthetics in cool running marine diesels glaze the cylinder walls and build up a very hard deposit on the edge of the piston crown that will score the cylinder. This subject has been researched fully and the results are easily available. However, don't take my word for it, read your engine's manual or get in touch with the maker. Or, to find out for yourself, run your engine for about 30 hours with a full synthetic and then check your oil consumption.
 

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Several years ago I was told by an aviator that switching to synthetic oil resulted in gaskets leaking, apparently because over time it replaced petroleum oil and the gaskets tended to shrink. My Mercedes was recalled for a switch from petroleum oil to synthetic. I agreed if the dealer would to handle repairs if gasket leaks developed. The dealer refused but assured me there would be no problem, I relented, and about two years later I ended up with a sizable expense for several leaking gaskets.

Was the aviator's story an urban legend? I don't know, but I don't have another explaination for what happened to me.
 

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While I am quite convinced that synthetic oil has some superior properties, namely lifetime, high temp stability, and viscosity vs. temperature, I don't think you're likely to take advantage of them. But worst case is you spend a bit more than you could have done.

Shell makes Rotella in standard, semi, and full synthetic versions. I refuse to believe that the synthetic version will destroy your engine when the standard version won't.
Do you still refuse to believe? I admire the man who sticks to his guns and refused to be confused be facts.
 

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I am perfectly willing to be persuaded by facts. So far all you have done is state some facts without providing any reference to back them up.

If you read that article linked to above, it does not state that you should avoid synthetics. It says that oils with high TBN can cause problems with cool-running diesels. So you should avoid oils with high TBN. That I can agree with, although I don't see any serious data to back this assertion up. But lets go with that hypothesis.

The article also states that the newest oils with a CJ rating, which are intended to be used with low-sulfur diesel, are good news because they can have a lower TBN number.

Here are some TBNs for commonly-used oils for marine diesels :

Chevron Delo 400 LE : 9.6
Mobil Delvac 1300 super : 10.5
Shell Rotella T non-synthetic : 10.1
Shell Rotella T6 full synthetic : 10.6

Are we to believe that going from a TBN of 10.1 to 10.6 will result in engine failure?

If we are to avoid T6 full synthetic because of a high TBN, it appears we also have to avoid any modern diesel oil.
 

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I read an article that blamed synthetic oil for causing the cylinders bores to be so. highly polished they couldn't be rehoned
Pure Bovine Scatology. I suspect someone read about not using it during initial break in because it's so slippery and didn't understand what they were reading.

Iron is so soft that it would not be possible to polish it to a point that a hone wouldn't cut it.

Synthetics are better than petroleum oil - pure & simple. The only downside to them is a possible slight increase in leakage - they are so slippery they can sneak out where conventional oil won't. They don't shrink gaskets, but even if they did you can buy "seal sweller" additives that would correct it.

Years ago, when they were still a specialty item I read a test where two identical engines (gas V8's), one on oil, one on synth, were run side by side on dynos for an extended period - the equivalent of 100K miles IIRC. After that they were torn down, inspected and measured. The oil engine demonstrated the usual wear for that much use. The synth engine was within OEM tolerances in all cases - essentially NO wear.
 

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Also it is my understanding that once you switch to synthetic in your engine you cannot go back to standard oil
Not true - you can even mix them if you want, or rather need to. A number of the "synth" oils out there are actually partial synthetics mixed with petroleum oil. You need to read the labels closely to see if that is the case.

There are still people out there who insist that "straight 30 weight" is the best thing for diesels. I think the anti-synth folks are their soul mates.

It's not necessary if you treat your engine properly but it IS better and it sure can't hurt. The references to "cool running diesels" means they either have incorrect thermostats or are raw cooled.

You aren't supposed to mix BRAKE FLUIDS and should flush the system completely when changing from one to the other.
 
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