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Old 12-08-2015, 09:06 AM   #5
DavidM
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City: Litchfield, Ct
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Why use a synthetic oil at all?


Synthetics have two primary advantages over a petroleum based oil: they have somewhat better lubricity compared with petroleum based multi grades and they don't oxidize as badly over time so they maintain their properties over a long period.


But petroleum based oils do just fine in the lubricity department and I suspect that you will never see any benefit during the life of the engine. Marine diesels don't wear out usually and never due to poor lubricity. Overloading, yes!


Secondly even ignoring manufacturer's recommendations, oil in diesel engines need to be changed to remove soot, fuel and replace lost additives, none of which is helped by having a synthetic. Sure big fleet operators test their oil and change it when those values get out of whack, but does a single boat operator want to do that, especially if it goes against the manufacturer's recommendations? So the low oxidation properties of synthetics is lost if you have to change the oil anyway.


But if you must use a synthetic I suspect there is no practical difference in Mobil 1's 15W50 and a petroleum based 15W40.


David
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