JackD
Veteran Member
I'm the new owner of a Mainship with Yanmar 6LYA-STP, 370HP. The owner's manual says I should use 15W-40 weight oil. The previous owner used 15W-50 Mobil 1 synthetic, which does not come in a 40 weight. Stick with the 50?
Mobil 1 15W50 is S rated for gasoline engines. You need to use a C rated oil which is formulated for diesel engines. If you want to stay with a synthetic I suggest Shell Rotella T6 5W-40 synthetic. If you don't want to stay with a synthetic then any CJ rated 15W-40 will do. Good brands are Shell Rotella T, Chevron Delo 400, Texaco Ursa, etc.
According to the product data sheet, Mobil 1 15W-50 meets the quality level of API CF.
Mobil 1â„¢ 15W-50
I only found the SM rating when I looked. I note that your reference says "meets the requirements for CF", but doesn't say it is CF rated. That said CF is an old rating and CJ would be much better. If the engine is 1998 or newer the manufacturer likely called for CH or higher rating oil. Here is the info for CF rated oil.
CF Obsolete Service typical of indirect-injection diesel engines and other diesel engines that use a broad range of fuel types, including those using fuel with high sulfur content; for example, over 0.5% wt. Effective control of piston deposits, wear and copper-containing bearing corrosion is essential for these engines, which may be naturally aspirated, turbocharged or supercharged. Oils designated for this service have been in existence since 1994 and may be used when API Service Category CD is recommended.
However, the statement of using C versus S rated oils stands.
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
The downside for synthetic in a recreational boat is hard to overcome.
Syntheic scrapes off and drains better , so it improves the mileage a tiny bit.
The hassle is they drain really well so the engine can get exposed to internal rust , and even dryer than normal cold starts.
Use the car or truck a lot , they do work, esp for ultra high loads in a high performance engine.
Use the boat once a month or so, there is no advantage.
On layup there is extra work required to seal the engine air tight.
I'm the new owner of a Mainship with Yanmar 6LYA-STP, 370HP. The owner's manual says I should use 15W-40 weight oil. The previous owner used 15W-50 Mobil 1 synthetic, which does not come in a 40 weight. Stick with the 50?
No practical difference between a 40 grade and a 50 grade oil? Then why make two different grades? The difference is the viscosity at operating temperature. Engines are designed to use oils of a specific grade. Using a 40 grade oil in an engine designed to use a 30 grade is just the wrong way to go. Some folks believe that a higher grade oil is better to use in higher ambient temperatures because it stays thicker. Yes, at operating temperature it does stay thicker, thicker than what the engine was designed for. Remember, regardless of the ambient temperature, an engine, barring extremes boaters do not see, will run at its design temperature as regulated by the thermostat. That means the oil will be at the same viscosity at operating temperature REGARDLESS of the ambient temperature. Hence, using the 40 grade oil is not good. At operating temperature, the 40 grade will be too thick. And don't quote me Bob Smith. He is not a petroleum engineer. This is science folks.
There is no good reason to run a 15W-50 in a sea water oil cooled Yanmar in trawler service.
Use any diesel rated oil you want.