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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 07-31-2013
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Confused about which oil weight to use

The shop manual for my Yanmar 2QM15 specifies either 10 or 20 straight weight oil, depending upon outside temps. I realize that when the manual was written, modern multi-weight oil didn't exist, but my understanding is that because the engine was designed to operate with 10 or 20 weight oil when at operating temperatures, I should use a multi-weight oil with a similar operating temperature weight (something like 5W20, for example).

Problem is that the readily available diesel oil doesn't seem to be available in this low viscosity. For example, Rotella T, which many recommend, comes in 15W40 and 10W30, both of which would seem to be too thick.

To add to my confusion, since this is a raw water cooled engine, it operates much cooler than a fresh water cooled engine, making the use of too thick of an oil even more of a problem.

What am I missing in this equation?
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Old 07-31-2013
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Confused about which oil weight to use

Use the mulit weight oil closet to the manufactureres recommended limits. Your on the right track.
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Old 07-31-2013
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Re: Confused about which oil weight to use

Strange that the manual calls for 10 or 20 weight oil. Most call for a 30 weight. I use Rotella 15W40 in my 3GM30F.
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Old 07-31-2013
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Re: Confused about which oil weight to use

Same here: Rotella multigrade in 3GM30F. Seems to work fine.
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Old 07-31-2013
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Re: Confused about which oil weight to use

The main difference is in the cold start... which in most pleasure craft will not be all that cold.

I've used Rotella T 15w40 in EVERY engine I've owned for the past 35 years. I get 250,000 miles out of every vehicle. But 97% of my miles are low rpm, seldom over 3000 and never over 3500. Even on the lawn mower I do not run wide open... 80% is my max. Once warmed up its in the 15 to 20 wt zone. 40 is at cold temps at whatever the standard temp is. They are all almost grease at below 0, but I highly doubt you'll be sailing at 20 below. Start and let it warm up before working as is proper for any engine and I expect you'd be fine. This doesn't work for some of the modern high rpm automotive engines though. They are a different animal.

Most of the marine sail applications on general use are likely low rpm high load... exactly what the Rotella 15w40 was designed for. It will keep a Mack 350 semi engine going at 100% for a million miles before a rebuild. Been there did that.

Now there is another potential issue though... that of detergent vs non-detergent oils. On many old engines pre 1950 or so that ran non-detergent is the shellac and gum that is actually doing the ring sealing. Switching to a detergent oil and they can start burning alot more oil. Its doubtful your in this category though.
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Old 08-01-2013
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Re: Confused about which oil weight to use

I recently switched to Shell Rotella 15W-40 in my Yanmar 2GM and it is now easier to start when cold. The recommended straight 30-weight oil seemed too thick for winter and loaded up the starter more (as you would expect). This engine is also raw-water cooled and never gets very hot. To flush out the old 30-weight completely I did an oil change after about three hours on the Rotella and now do it twice a year, regardless of hours run (my engine does big hours by pleasure boat standards).
It is said that any damage to an engine happens in the few seconds after starting, before the oil gets to all the bearings etc, so lightweight has to be a good thing. Even though the manufacturers always want you to use their own expensive 'home' brand oil, I have run a fairly cheap locally-produced 15W-60 semi-synthetic diesel-engine oil in air-cooled dry-clutch motorcycles for many years with no issues.

With oil, you will get dozens of different opinions, sometimes clouded by a commercial interest. Mine is that any oil these days has to be reasonably good to make it to market and I would place more importance on twice-yearly oil changes than any particular brand. Shell Rotella is as good as any, but if I couldn't get it I would buy whatever grade and brand suited the engine. My other belief is that there is nothing wrong with mineral oil in a marine application, and in fact, full synthetic is probably a waste of money (ducks for cover.....).

If it is recommended to use straight 30 in your gearbox, stay with that. Unless it uses something else, like hydraulic fluid, of course!
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Old 08-01-2013
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Re: Confused about which oil weight to use

i was told by my yanmar distributor that i should use 30 weight in my yanmar, and
40 in my perkins.

so--yannie gets 30 wt oil and perkie gets 40.
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Old 08-02-2013
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Re: Confused about which oil weight to use

I have used the 15W40 MotorCraft from WalMart. I hope you all don't blow a gasket at the oil I use. My motor hasn't yet. But, the specs read that it exceeds standardized oil
at that SAE. Does anyone have any experience with these matters that should advise me otherwise? I have a Island Packet 32. A 3GMD yanmar (20 HP) was put in, in place of
the previous, fresh water, coolant (24 HP). Does anyone know if this is too small of an engine for a 22,000 IP that is 32 feet long? thanks. New to Sailing.
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Old 08-02-2013
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Re: Confused about which oil weight to use

As long as the Motorcraft has the right rating, it's a Ford product and probably ok. You might want to shy away from the generic Walmart brand although they are probably all ok in reality. I'd be a bit leery about an off brand's QC and would worry that what it says on the can might not be what is actually inside. Could be a REAL expensive error to save a couple of bucks. I use Pennzoil in my cars and trucks. My last Subaru was still running fine at 365,000 miles when I traded it. Changing oil religiously in cars or boats, diesels or gassers is the key to long engine life. With Yanmar, it's 100 hours which comes around pretty fast. I never go cruising without at least 3 oil changes and filters on board.

The old CD rating is listed for Yanmar diesels. It's not really available anymore but the HEAVY DUTY DIESEL C ratings are what you need to look for.http://www.api.org/certifications/en...010_120210.pdf Light automotive diesel oil won't cut it.

That's a perfect engine for your boat.
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Last edited by smurphny; 08-02-2013 at 06:59 PM.
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Re: Confused about which oil weight to use

Thanks Smurphny,
Last week, I ran the motor in idle, in neutral for about 25 minutes, it had plenty of water flow in exhaust, Then, I thought it might be getting too hot,
So I reved it to 2500 for 15 seconds, then lowered RPM to idle in 5 seconds,
and shut it down. For some reason, the motor was very HOT, and it froze for about 2 hours. Since, I did not know it froze, I tried to start it after about 20 minutes after I shut it down, and It blew the starter aluminum cast shroud over the gear teeth into two pieces. Luckily, since this happened once before,
I had bought two, to have a spare, and replaced it. No aluminum cast pieces went into the flywheel cavities. But, does anyone know why it got so hot,
while running in idle?
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