Oil for a Yanmar raw water cooled sailboat diesel

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Rusty123

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This may not be the best place to post, but the marine forum doesn't seem to address small diesels.

I have a sailboat with a 1981 Yanmar 2QM15 diesel (2 cylinder, 15 HP). Engine is raw water cooled, so full operating temperature is only 125F. I sail in the Puget Sound, so temperatures are fairly moderate year round (30F - 90F most of the time).

Shop manual specifies straight weight oil, as follows:

50F - 60F: 20 weight
60F - 95F: 30 or 40 weight
>95F: 50 weight

Manual also specifies API CD oil. CD oil appears to be obsolete, but I've come across some discussions on the web that state that newer oils can be detrimental to small, low temperature marine diesels like this, primarily because of the high TBN value (apparently, small raw water cooled marine engines don't generate enough acids to offset the high TBN in modern oil, so the resultant alkalinity of the oil causes its own set of problems).

Any advice regarding oil selection? 15W40 seems to be in the sweet spot for viscosity, but the API CD thing has me stumped.
 
I'd say use a 15w40 diesel oil. APIcd was the spec back in 1981. The first letter stands for compression and the second one is the spec. Right now, we're at CJ-4 Oh and the 4 stands for 4 stroke.
 
What's available that would be the closest to a CD addpack? PCMO SN 10w30? It shouldn't be a TBN superstar that a modern C-rated oil would be.
 
I run 15W-40 diesel motor oil in my Westerbeke 21A 3 cylinder. It is the type most recommended.
 
Any of the 15w40 brands. If you are worried about the engine temp...change the thermostat.
 
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I'd use something fairly thin, since with raw water cooling, the oil will never reach operating temperature.
In a currently available oil, T5 10W-30 would be a good choice.
If you can find a diesel rated SAE 20, that would be a good choice as well.
Ignore those who recommend 15W-40 or SAE 30.
Those oils are too thick for the water temperatures you see.
 
Since the manual calls for 50 weight when air temps are above 95, that tells me the engine actually prefers 50 weight at operating temps (which are controlled by the T stat), and that lower viscosity oils are only necessary to ensure cold start flow. Since multi-weight oils provide that low temp flow, shouldn't I be looking for an oil with a upper rating of 40 or 50?

Confused.
 
Originally Posted By: Rusty123
Since the manual calls for 50 weight when air temps are above 95, that tells me the engine actually prefers 50 weight at operating temps (which are controlled by the T stat), and that lower viscosity oils are only necessary to ensure cold start flow. Since multi-weight oils provide that low temp flow, shouldn't I be looking for an oil with a upper rating of 40 or 50?

Confused.


Exactly. You could use a 5w40 but I think that only comes in synthetic.
 
Originally Posted By: motor_oil_madman
Exactly. You could use a 5w40 but I think that only comes in synthetic.


Good, so I'm on the right track. Aside from price, I too like the idea of the 5w40 (like Rotella T6), because this engine is a little tough to start, since it doesn't have glow plugs. I'm thinking the extra cold flow might make starting a bit easier, particularly in the winter.
 
I asked a reputable Yanmar dealer, Torresen Marine, the same question about 16 years ago. Their response to me was that they use 15w-40 in everything. I've used that ever since then with no problems or issues.
 
Originally Posted By: svchareta
I asked a reputable Yanmar dealer, Torresen Marine, the same question about 16 years ago. Their response to me was that they use 15w-40 in everything. I've used that ever since then with no problems or issues.


I picked up some 15W-40 Delo 400LE at Costco at a pretty good price. Looks like that's the ticket. Thanks to all.
 
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I would recommend 10w-30 dino HDEO. Good vis spread for your expected temps. 40 grade is probably too high. The new CJ-4 lubes have a lower TBN if that is what you seek, relative to the 12-16 of CH and CI lubes.
 
Originally Posted By: dnewton3
I would recommend 10w-30 dino HDEO. Good vis spread for your expected temps. 40 grade is probably too high. The new CJ-4 lubes have a lower TBN if that is what you seek, relative to the 12-16 of CH and CI lubes.



Good news on the TBN - that concern is lessened. But I'm confused by your viscosity recommendation. Since the manual recommends straight 50 weight when cold starting is not an issue, and since multi-weight oil essentially makes cold starts a non issue in my climate, why wouldn't I use a thicker oil - 40 or even 50 wt?
 
You can get Shell T1 SAE30 or SAE40 which is a CF/CF2 oil, that is as close to API CD as you can get.

However, I do believe that the more modern oils are superior.
 
Originally Posted By: Rusty123
Originally Posted By: dnewton3
I would recommend 10w-30 dino HDEO. Good vis spread for your expected temps. 40 grade is probably too high. The new CJ-4 lubes have a lower TBN if that is what you seek, relative to the 12-16 of CH and CI lubes.



Good news on the TBN - that concern is lessened. But I'm confused by your viscosity recommendation. Since the manual recommends straight 50 weight when cold starting is not an issue, and since multi-weight oil essentially makes cold starts a non issue in my climate, why wouldn't I use a thicker oil - 40 or even 50 wt?


I was looking at your first post; shows 10 grade down below 50F to 30 grade up to 95F. I presume that's the water temp? 10w-30 should cover 99% of what you'll see in WA.
 
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