Oil Pressure Fluxuates--Should I Change Viscosity?

[Mercedes_220_8]

Hi All,

I know this topic has been addressed here and there, but I'd really appreciate advice specific to my situation.

I have a 1970 Mercedes 220, gas, 4 cylinder, Stromberg carburetor. I recently acquired the car, and changed the oil for the winter (California Bay Area) with Redline 10w30. Of course the next day was 80 degrees, and I took it for a long drive out at the coast. After it was thoroughly warmed up, the oil pressure drops at idle speed from 45 (top of gauge) to between 30-35 when idling.

So my questions:

(1) Should I be concerned about this--what are the risks?

(2) Should I normally be running 20w50, even in the winter (when it never really gets below 30F)?

(3) And finally, should I pull a quart and replace it with a quart of 20w60?

The car's manual calls for SAE 30.

Thanks to all, looking forward to being a member of this forum. Cheers

 

[JGW]

No, that's normal.

 

[bruno]

First things first - confirm the oil pressure with a shop gage , second - that's an old car with how many miles on it ? It may be worn out and playing with oil grades isn't going to help .

 

[TFB1]

It's normal for oil pressure to be less when hot, 30psi at hot idle is actually very good...

 

[Oldwolf]

Oil pressure always fluctuates with engine RPM in a hot engine. So you are OK. Anyway 30-35 psig is high for idle. Ditto confirming the oil pressure though.

 

[SubLGT]

30-35 psi is a high oil pressure when idling a warmed up gas engine. What oil pressures does Mercedes specify in the service manual?

 

[Jim Allen]

No worries.

To me, first thing is to look up normal oil pressures for that car and engine. I used to work on those cars professionally back in the day and my memory is fuzzy on exact specs, but the pressure you state sound fine to me. Oil pressure drops because the oil thins as it warms up. That's perfectly normal for any car.

If the the car is rated for 30 grade oil by MB, then a 10W30 is fine. I would NOT use a 20W50 oil in that car. Remember that pressure is resistance to flow... ie pressure is just the "push" for flow. You don't really need any more than the spec listed in the manual.

 

[Brit33]

wow I wish I had 30 psi at idle hot.

I have 9-10 psi with my alfa romeo.
As there is no load on the engine at idle it is not so much of a concern unless it is so low that it triggers the low pressure sensor.
rule of thumb is 10 psi per 1000 revs.

What oil pressure are you getting at normal-high revs?

 

[TallPaul]

Wish my oil pressure would fluctuate, but I have the stupid Ford better idea idiot gauge that is either off or on.

 

[eljefino]

Questions like this are why they deadened oil pressure gauges so they aren't very effective or accurate anymore.

And to OP you are fine. Sounds like a nice ride.

 

[Gokhan]

Originally Posted By: Mercedes_220_8
Hi All,

I know this topic has been addressed here and there, but I'd really appreciate advice specific to my situation.

I have a 1970 Mercedes 220, gas, 4 cylinder, Stromberg carburetor. I recently acquired the car, and changed the oil for the winter (California Bay Area) with Redline 10w30. Of course the next day was 80 degrees, and I took it for a long drive out at the coast. After it was thoroughly warmed up, the oil pressure drops at idle speed from 45 (top of gauge) to between 30-35 when idling.

So my questions:

(1) Should I be concerned about this--what are the risks?

(2) Should I normally be running 20w50, even in the winter (when it never really gets below 30F)?

(3) And finally, should I pull a quart and replace it with a quart of 20w60?

The car's manual calls for SAE 30.

Thanks to all, looking forward to being a member of this forum. Cheers

Think of the oil pressure as your blood pressure -- lower is better, unless it is abnormally low.

Oil pressure is only important for oil flow: contrary to what some people think, it has no direct effect on lubrication and wear protection. Therefore, do not try to maximize your oil pressure, which is actually bad for your seals and oil pump (in analogy to your heart and veins), and it might be a sign of engine malfunction or a clogged oil filter or too high oil viscosity, which results in too low an oil flow. As long as there is no engine malfunction, lower the oil pressure, the better it is for the engine. Use the thinnest oil viscosity recommended for the engine for normal driving, which will result in the lowest oil pressure. Thinner oil will also result in less deposits and clean the existing deposits better (in analogy to blood and cholesterol in your arteries).

Your oil pressure seems to be too high at idle. You might have restricted oil passages or a clogged oil filter.

If it recommends SAE 30, for best performance, use an affordable fully synthetic 5W-30 or 0W-30. You can also use a conventional 10W-30 if it's consuming a lot of oil and doesn't make sense economically to use fully synthetic because of high consumption. Since it specifies straight SAE 30, do not use a conventional 5W-30 because it shears a lot. Conventional 10W-30 or fully synthetic 0W-30/5W-30/10W-30 are fine.

The best choice, except for the cost, would be a fully synthetic 5W-30 or a fully synthetic 0W-30, which will also help clean your restricted oil passages and last longer in your old, somewhat vintage engine. I would recommend Mobil 1 0W-30 or Mobil 1 5W-30.

 

[Brit33]

Quote:
Think of the oil pressure as your blood pressure -- lower is better, unless it is abnormally low.


Well you want a medium pressure - wouldn't go as far as saying the lower is better. That is misleading.
Have a look at when your oil pressure excess valve opens.
Something below that level is good.
Mine begins to open at around 65 and is fully open at 88.
Also what does the manual say? Follow the manufacturer's recommendations as a guide.

My max oil pressure is about 55-58 psi at full revs.
Normal driving, 3000 revs for example, it sits at a happy 40-45 psi.

Quote:
Your oil pressure seems to be too high at idle. You might have restricted oil passages or a clogged oil filter.

this is where we need more info about pressure at different rev ranges.

 

[Trav]

Quote:
Think of the oil pressure as your blood pressure -- lower is better

Is it going to bust an artery or what?

In what way is it better? Any documentation other than something by AEHaas?

Edit: the oil pressure for that engine at idle is okay 30-50 psi.

 

[Brit33]

Quote:
Is it going to bust an artery or what?


well a very high pressure can bust seals/filters in the way arteries burst.

But yes have to agree lower is not always better.
Again check manufacturers recommendations.
Every engine is different.

 

[Gokhan]

Originally Posted By: Trav
Quote:
Think of the oil pressure as your blood pressure -- lower is better

Is it going to bust an artery or what?

Actually the analogy between the blood circulatory system and the engine lubrication system is quite good in many ways.

It could bust your seals or oil pump.

Quote:
In what way is it better? Any documentation other than something by AEHaas?

The important point is that the oil pressure has no direct effect on lubrication. If the oil flow is the same, your bearings, cylinders, and valvetrain will get the same lubrication. The complex formula for the minimum oil-film thickness (MOFT) does not involve the oil pressure.

If you use a thinner oil, the oil pressure will go down. That doesn't mean that the oil flow is decreasing; on the contrary, it's either increasing or staying the same. MOFT will decrease with decreasing viscosity but oil pressure itself has no effect on that. In other words, if you replaced your oil pump with a high-pressure oil pump, it would have no effect on your MOFT, as long as you are using the same oil (viscosity). You could benefit from more oil flow at lower RPMs at the expense of more strain on your seals, but that would be it.

 

[MarkStock]

Redline in 30 weight is more like a 40 weight.

If you want to use Redline for a car that asks for 30 weight, consider going for their 20 weight oil.

Otherwise, consider picking a 30 weight from another brand.

 

[Gokhan]

Originally Posted By: Brit33
Again check manufacturers recommendations.
Every engine is different.

Yes, but manufacturers do not recommend an oil pressure -- they only recommend an oil viscosity. It's a good idea to stick with the viscosity recommendation to ensure a sufficient MOFT etc.

The repair manuals do specify wide oil-pressure ranges, but these are only to determine if there is a malfunction in the lubrication system, not to optimize the engine performance. There is a maximum value ensured by the relief valve to protect the engine, which shouldn't be exceeded, and the minimum value at the specified RPM is to check that the oil-pump is healthy and there are no leaks, restrictions, etc.

 

[Brit33]

Maybe we need to be concerned with Mercedes_220 oil pressure at higher revs.

Can you tell us what the readings are at high revs?

 

[CATERHAM]

First welcome to BITOG!

Yes your oil pressure reading (which actually is a measure of the back pressure in the system) sounds perfectly fine. If you can, try to find out what the factory test OP spec's are for your engine.
The Red Line 10W-30 is equivalent to a fair robust 40wt oil, so in effect you are already using a heavier oil than what's spec'd for your engine.
Is you like RL I'd go with their higher VI 5W-30 next time or even their 0W-40.
Another cheaper option would be Mobil 1 0W-40 but your OP readings will be somewhat lower with this oil.

 

[Brit33]

Quote:
Yes, but manufacturers do not recommend an oil pressure -- they only recommend an oil viscosity


totally untrue.

Alfa Romeo specify oil pressure for idle and max revs.