Old engine: 30 weight or 40?

[Naviget]

I have a 1979 Yanmar 3QM30. Don't know how many hours (I am 3rd owner). Religiously change oil using Rotella T1 30 weight. At engine start, pressure is 58 psi for a few minutes. Running at 2300 rpm all day on ICW, drops to 31 and holds for a few hours, and then drops and holds around 23 psi. Manual says pressure at this RPM should be around 40 rpm, dropping to as low as 7 psi at no load idle. Manual says that I can choose 30 or 40 weight with ambient temp in the 70-95F range. Engine compartment has no ventilation and hovers between 127F and 160F.



Would 40 weight have advantages in this application over 30 weight?

[motion30]

40 weight with most likely bring your pressure up a little. If you haven't electric gauge I would check it's accuracy with a mechanical gauge. No sense working on the problem if it does not exist

[tkeithlu]

This appears to me a good situation for using a multi-weight oil, now that they are approved for diesel engines. A 10W40, for example, would give you fluidity at low temperature and adequate viscosity at high temperature. Just be sure that you are getting a "C"rated (diesel) oil.

[Rumrace]

Oil like many things has made a leap forward. On an older engine like yours I would specifically use Quaker State High Milage 10w 40. This oil is wonderful for restoring old engines.
That said you have an oil leak somewhere and a very tired oil pump. I’d guess the leak is the pump.
I use Castrol on just about everything. Transmissions or hydraulic oil Royal Purple Oil. Penzoil has parrafin in it which will clog the oil passages in the crank when cold in a Yanmar.
Products like duralube put additives in a litre of 30 w oil. Quaker State has put these poly additives in good multiweight to fill scratches maintain pressure.

[Naviget]

Thanks for the tips. I like the 10W-40 suggestion, and appreciate the input on brands.



I did have an issue with the electrical sender, and did tee in a mechanical gauge, therefore am reasonably confident of the pressure readings finally.

[Lepke]

Make sure whatever oil is diesel rated.

If it was my engine, I'd change the oil pump.

[Engine man]

Your engine manual gave the correct information back when the engine was new. However, in 2022, there are better oil alternatives. We suggest a multi weight oil, such as Shell Rotella 15W40 for year round use. Your engine will be easier to start, [ the oil will be less viscus when cold, due to polymer additives] and will become more viscus when the engine is at operating temperature. [ resulting in higher oil pressure, and higher cylinder compression] Joe

Originally Posted by Naviget

I have a 1979 Yanmar 3QM30. Don't know how many hours (I am 3rd owner). Religiously change oil using Rotella T1 30 weight. At engine start, pressure is 58 psi for a few minutes. Running at 2300 rpm all day on ICW, drops to 31 and holds for a few hours, and then drops and holds around 23 psi. Manual says pressure at this RPM should be around 40 rpm, dropping to as low as 7 psi at no load idle. Manual says that I can choose 30 or 40 weight with ambient temp in the 70-95F range. Engine compartment has no ventilation and hovers between 127F and 160F.



Would 40 weight have advantages in this application over 30 weight?

[wingssail]

I have exactly the same problem with my Yanmar 3JH showing 6900 hours. My oil pressure dropped from 45 to below 20 after a few hours of operation, and that concerned me. I too Tee'd in a mechanical guage which confirmed the pressure drop. There were other symptoms which also concerned me. A few consultant visits by mechanics finally convinced me the engine was OK, just a little worn out (tolerances, etc).

I changed to Castrol GTX 20-50 (we operate in a hot climate) and my oil pressure worries have gone away. My oil gauge now registers over 50 at cold start and rarely goes below 45. I have also confirmed that oil is flowing to the rocker shaft, which I could not tell for sure previously.

[JPA Cate]

Originally Posted by wingssail

I have exactly the same problem with my Yanmar 3JH showing 6900 hours. My oil pressure dropped from 45 to below 20 after a few hours of operation, and that concerned me. I too Tee'd in a mechanical guage which confirmed the pressure drop. There were other symptoms which also concerned me. A few consultant visits by mechanics finally convinced me the engine was OK, just a little worn out (tolerances, etc).

I changed to Castrol GTX 20-50 (we operate in a hot climate) and my oil pressure worries have gone away. My oil gauge now registers over 50 at cold start and rarely goes below 45. I have also confirmed that oil is flowing to the rocker shaft, which I could not tell for sure previously.

You know, I, too thought of Castrol 20-50. But the OP lives in New Jersey, and it freezes there, so maybe the 10-40 would be more suited. Also, I thought Lepke's suggestion to rebuilt/replace the oil pump might help a lot for a tired engine.

Ann

[Compass790]

I'd check oil pump tolerances as per manual if thats possible I didnt check the applicable w/shop manual. For example of higher oil pressure with viscosity change I get about 5psi hot increase going from 15w-40 to 20w-50. If your oil pump tolerances are good the next suspects will be worn mains or big end bearings.
Personally I'd run 20w-50 CF or CH in that motor, even if it freezes you are better off than mono 30 or 40. Your oil pressure is a bit low but could go for a decade or more like that without issues.
Really what winggsail & Lepke said.

[Jammer]

Been there, done that with many ag diesels from that era.

Originally Posted by Naviget

Would 40 weight have advantages in this application over 30 weight?

It really doesn't make much difference. A worn engine is a worn engine and oil pressure and lubrication will suffer and before long it will be shot no matter what kind of oil you put in it. If it's at the point where you're thinking about switching viscosity it's probably at the point where there will still be a problem after you do.



Diesels last a long time and you might get 1000s of hours before it is really shot.



With heavier oil you will get marginally better oil pressure, marginally less leakage and oil consumption, and marginally worse cold starting performance.


I used to get 15w-40 by the 55 gallon drum and more or less used it in everything. I experimented with 20w-50 in older crap engines and 10w-30 in the winter but it didn't matter much.

[JimsCAL]

Sounds like some people are using oils certified for gasoline engines. Not a good idea as the additive packages are different. Use an oil certified for diesels like Shell Rotella T4.

[Celestialsailor]

Originally Posted by Rumrace

Oil like many things has made a leap forward. On an older engine like yours I would specifically use Quaker State High Milage 10w 40. This oil is wonderful for restoring old engines.
That said you have an oil leak somewhere and a very tired oil pump. I’d guess the leak is the pump.
I use Castrol on just about everything. Transmissions or hydraulic oil Royal Purple Oil. Penzoil has parrafin in it which will clog the oil passages in the crank when cold in a Yanmar.
Products like duralube put additives in a litre of 30 w oil. Quaker State has put these poly additives in good multiweight to fill scratches maintain pressure.

Here you go again spewing misinformation about something you obviously know nothing about. The oils you list are detergent oils which are not recommended or good for diesel engines. There are oils from reputable manufactures that are recommended specifically for diesel engines. Some marine diesel engine manufacturers recommend different oil manufacturers.

[Rumrace]

Some folks contribute some folks talk smack.
Castrol GTX is awesome. It was originally designed to protect engines which just lost the lead in Gas. I worked for Burma Oil ( Castrol ) when it was first packaged. It’s not for diesels but probably keep the barrels and rings well coated and the crank acid free or balanced. Consider GTX is designed for 6500 rpm motors.

[AiniA]

I also have an old engine (Volvo MD 11C). Would there be any advantage to using a (diesel-specced) synthetic oil rather than conventional?