Old engine: 30 weight or 40?

[Rumrace]

We packaged hypoid 80/90 as motorcycle fork oil in 1/2 pints for 10X the price
Vedoil muktigrade for marine for more dollars than a much better GTX.
We packaged BMW oil which was GTX they sold with 60% margin.
People who buy wax oil brands cause they have “ marine “ on the label are a wonderful bunch.
Quaker State included your wee Yanmar in their repair oil warranty

[Rumrace]

Did you know the Toronto Star did an independent test on motor oil 2 decades ago. They tested oil to SAE standards at 5,,000 miles. Castrol, Quaker State, Mobile passed. Shell Exon failed with 20 other brands many private label.
At 10,000 miles Castrol and Quaker State still Met SAE standards.
Every single brand has increased in quality.
Marketing same same.
Many products UL SAE ETL approved don’t measure up at end of life. Oil is one thing we can’t allow to slip away. We all ready produce the worst fuels.

[Rumrace]

Synthetic oil doesn’t take prisoners. If there is a spot to leak it seems to find it.
If the motor is clean gaskets are tight go ahead. It clings to metal rejects contaminates like no other oil. Every oil I’ve suggested so far is a synthetic.
How Royal Purple Hypoids work so well is a mystery but your transmissions drive will run cooler and they won’t swell like let’s say Mercury branded.

[sailorboy1]

Originally Posted by Jammer

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.

I was thinking this the whole way reading to this point so backed up to agree.

But that doesn't mean you can't get a lot more life by using the heavier oil. In the end what is there to lose IF the wear is already there. The real question for the OP to think about is whether the low pressure is from bearing clearances, the oil pump, or the oil temperature.

Given it takes hours it sounds for the pressure to drop to me that would suggest it isn't acturally a clearance or pump problem. So maybe what it really is is a cooling problem. A heavier oil would address a cooling issue, for a while. But I bet a oil cooler cleaning or upgrade would be the way to go.

[JimsCAL]

I am careful about suddenly switching to full synthetic oils on old engines. To many reports deposits being loosened and causing problems. The Rotella T4 is a synthetic blend and would be a good compromise.

[Rumrace]

I agree with both of those comments. Oil sounds like the least of the problems. Best place to loose oil pressure is in or around the pump. Sure fire loss of oil pressure is crank slapping around. A new Yanmar is $4,700.00. Well priced
But yes a synthetic multigrade is what you use in a modern Kubota tractor, sorry marine diesel.

[MJH]

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?

If I were you I would contact Yanmar, tell them what model engine I had and hours on the engine, if available, and ask them what they think. I use Chevron Delo 400 15W-40 or Rotella T 15W-40. A single weight oil doesn't cover it all.

Rotella T1 is an oil for diesel engines, so that is a good start. When I bought my boat it came with a professionally installed brand new diesel engine (Volvo Penta D2-55B) and transmission. It replaced the previous owner's Perkins 4.108. What I found onboard was about a case of Texico Havoline 10W-30 oil for GASOLINE engines...no wonder it needed replacing. I used that oil in my lawn mower.

[Celestialsailor]

Originally Posted by Rumrace

Some folks contribute some folks talk smack.

Ya...over 800 posts in 3 months...

[Rumrace]

It’s real easy if the label says “ C” you can use it on compression motors. If it has “S” it’s for service or spark. Don’t use S in diesel. Better yet don’t use C in a gas motor it can’t push the massive additive package S doesn’t get.

[bongo]

All internal combustion piston/ring type engines have some blow-by. With the sulfur in Diesel fuel that blow-by slowly accumulates in the sump and will eventually turn the lubricating oil acidic. “C” type lube oil, rated for Diesel engines, has an additive package that enhances the TBN, (total base number), of the lubricant, enhancing its ability to not become acidic. Eventually even this “C” type oil will become acidic, (though not as quickly as a type “S” lube oil), this is one of the main reasons to change the lube oil. On ships the lubricants pH is monitored, and renewed as necessary.

[sailorboy1]

So far I am the only one questioning why/how the oil pressure would go down after hour plus. It isn't like after that time the engine or oil pump clearances got bigger after that.

[Cheechako]

If multigrade is recommended as an option in the manual I would use that. But if it has not been used most of the time in the past, hmmmm... maybe not. It works well and keeps engines cleaner if used extensively. Old school oil gums up engines badly.
maybe 'if it aint broke dont fix it' applies here.

[Rumrace]

Nice explanation Bongo thank you.

I think the worst thing to do is put a boat away for the winter without changing the oil.
You’d hope the drop in pressure is the oil pump swelling or something easily restorable. It’s anyone guess.
Like someone posted it could be at any bearing or bushing.

[Rumrace]

Great article on how diesels loose oil pressure. You’ll note debris is the number one reason.

https://www.capitalremanexchange.com/7-reasons-for-low-oil-pressure/

[Compass790]

Originally Posted by sailorboy1

So far I am the only one questioning why/how the oil pressure would go down after hour plus. It isn't like after that time the engine or oil pump clearances got bigger after that.

I thought you made a fair point but it takes our engine about an hour to get to its lowest oil pressure & fully warmed up & it definitely has no cooling problems. Its raw water cooled & oil temp never gets above 60oC.
The clearances do not change but the higher the oil temp the less viscosity= less pressure as it flows faster. Thats why oil pressure is so high when cold & drops as it warms
Worth checking the cooling as you suggest especially if its never been flushed.
Given the engines age I'd suspect bearing or oil pump wear unless its low hrs

Just noticed your engine compartment temp is 160 F! thats pretty high for a raw water cooled engine. I'd get some ventilation rigged up & I'd expect an increase in oil pressure with better engine box cooling

Whoops I see it takes 3hrs to get to lowest oil pressure re reading OP.