oil pressure, what's good

[peter230e]

Hi All,

Thanks to the quick responses on this web I have learnt a lot already. Think I will take the GC (ow-30) for my mercedes 230e 1983. Still doubting a bit because mercedes and sheel and castrol say to stick to the original 40 grade in the manual. That brings me to this question:

I read that lower visc is better for lots of things: temp, flow, fuel consumption, drag etc etc. Are there any down sides of going down one grade?

Mercedes says to stick to the old manual and just use mineral 15w40. Afetr reading this site I am tempted by Full Synthetic 0w-30, the German Castrol which is almost as thick as a 40. Oil pressure will go down a bit maybe, what is the disadvantage? The higher the better?

Someone at Castrol said to me that oil also leaks through the engine and that thinner oil leaks faster and does not stick as it should like the 40. In other words not everywhere in the motor exists oil pressure and the 30 will leak faster down than it should with the advised 40. Can anyone further explain this? Up till now I got the impression that thinner is always better.

How does one know what a good oil pressure is. What is the danger of an oil pressure that is to high or to low?

Thank you all
Peter

 

[Tempest]

Quote:
Someone at Castrol said to me that oil also leaks through the engine and that thinner oil leaks faster and does not stick as it should like the 40. In other words not everywhere in the motor exists oil pressure and the 30 will leak faster down than it should with the advised 40.

Doesn't make any sense to me but I'm not familiar with the great many variables involved.
Quote:
Up till now I got the impression that thinner is always better.

Thin as practical yes. BUT, bearings and other clearances are engineered for an "ideal" viscosity. Too thin an oil in a large clearance bearing is not good. HTHS is a good gauge for this.

 

[660mag]

I have always been told that 10psi per 1000rpms while under load is truthfully more than enough. That's what I've been taught.
How ever I have no proof that that holds water.

Anyone?

 

[Corvette Owner]

Originally Posted By: 660mag
I have always been told that 10psi per 1000rpms while under load is truthfully more than enough. That's what I've been taught.
How ever I have no proof that that holds water.

Anyone?


I agree that this was the "old" rule-of-thumb and is more than enough.

 

[CATERHAM]

Yes 10 psi/1,000 rpm at higher rev's but 15psi/1,000 on idle and up to 3,000 or 4,000 rpm. These are ball park minimum figures.

Ideally one should refer to the factory min' OP spec's that are in the work shop manual.

 

[peter230e]

Ok, thanks so far. How does psi compare to bar? The min OP spec of my manual says 0,5 bar at idle with a hot motor. Mine is 1,7 at idle. This is very, very normal however for these cars. Almost all w123 owners have this pressure and say its very good.

But would it make sense to get a little nearer to the o,5 bar of the manual by using the GC instead of a normal 40? I know the o,5 is a minimum, but I cant figure out what the ideal pressure is.

Would it make sense to say that the advises 40 grade includes use like towing and really hard driving and going into desert like landscapes etc? And that for that reason I could better go down a grade since I am always driving carefully, not to fast, no towing etc?

Learning a lot! Thanks

 

[wolfc70]

Originally Posted By: peter230e
Ok, thanks so far. How does psi compare to bar? The min OP spec of my manual says 0,5 bar at idle with a hot motor. Mine is 1,7 at idle. This is very, very normal however for these cars. Almost all w123 owners have this pressure and say its very good.

But would it make sense to get a little nearer to the o,5 bar of the manual by using the GC instead of a normal 40? I know the o,5 is a minimum, but I cant figure out what the ideal pressure is.

Would it make sense to say that the advises 40 grade includes use like towing and really hard driving and going into desert like landscapes etc? And that for that reason I could better go down a grade since I am always driving carefully, not to fast, no towing etc?

Learning a lot! Thanks


1 bar is roughly 14.5 PSI. So you are seeing roughly 21 PSI at hot idle, which is well above the minimum you stated (.5 bar or about 7 PSI). I would not hesitate to use a thinner oil. If you see a drastic drop in oil pressure at idle, then I would change back. If the engine is healthy, I doubt you will lose that much oil pressure, my guess would be less than 0.5 bar drop at hot Idle.

 

[Jim Allen]

The factory oil pressure specs are the best place to start, though you may have to dig deep to find them. These pressures usually apply for a hot engine using the OE specified viscosity but there may be special conditions listed.

For older engines, the old adage of 10 psi/1000 rpm was the MINIMUM, get-you-home-alive spec. IMO, it didn't/doesn't apply to all engines and is less applicable to newer stuff based on what I have seen.

My 5.4L Ford would be about dead with pressure that low because the VCT (Variable Cam Timing) uses oil pressure (5W20 @ 210F = 60psi + oil pressure @ 2000 rpm and about 26 at idle). It has a crankshaft driven, Gerotor style pump. With various types of VCT and Gerotor pumps becoming industry norms, it's a new oil pressure ball game out there and the old rules don't always apply.

If you wonder, as I did, install an accurate oil pressure gauge and oil temp gauge. I've done that with four engines now and learned a lot, It's enabled me to make small changes in oil viscosities with no worries.

Specifically to the OP, I'd think any changes in pressure from 40 to a thick 30 would be minimal, and would depend on how thick (in centistokes.. a "40" grade has a wide range of actual viscosities and it could be on the thin side of the grade). In two of my Ford diesels (one a tractor one a pickup), for example, dropping down from a 15W40 (on the thick side of 40) to a 10W30 (that's just a coupla notches below 40 anyway) cost 3-4 psi and left me well above the minimum values listed in the manuals.

 

[Astro14]

In that vintage 123 body, I think M-B specified 0.5 bar minimum pressure at idle (hot). I rarely see below 1.0 bar on the 300E when hot and it quickly goes up to 3.0 bar when engine RPM climbs above idle.

I would stick with the M-B recommended viscosity for temperature. I've found the 5W40 Rotella T6 works great in my climate...which is pretty similar to yours...but the GC is very close. Just make sure you're above that minimum idle pressure in the manual.

 

[CATERHAM]

Originally Posted By: Astro14
I've found the 5W40 Rotella T6 works great in my climate...which is pretty similar to yours...but the GC is very close. Just make sure you're above that minimum idle pressure in the manual.

GC (HTHS 3.5cP)is a much lighter option to Rotella 5W-40 (HTHS 4.0cP).
The obvious "best" choice is readily available Mobil 1 0W-40 (HTHS 3.8cP) and with it's 185 VI is even lighter than GC (VI 167) on start-up.

 

[mechtech2]

That Castrol is pretty thick.
More like going down 1/4 to 1/2 grade, depending.

I save big $ and use the 15w40 conventional.
Cold starting next winter? This could be a problem so a synthetic may be smart, or a thinner viscosity [especially the FIRST # - instead of a 15, a 0 or 5]

 

[CATERHAM]

Originally Posted By: mechtech2

I save big $ and use the 15w40 conventional.

Big $s give me a break.
You'd have to pay me a lot of money to even think about using such an antiquated lube in any car or truck I valued.
When you can get such vastly superior oils like Mobil 1 0W-40 and Rotella T6 5W-40 on sale for 5 bucks a qt it's clearly false economy to use a 15w40 conventional even in the summer.