Low Viscosity = low oil pressure?

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No, it depends on pump design, and if the pump and bearing clearances are OK. When swapping from a 20W-50 to 10W-30 to 0W-5 on a 1600CC engine I experienced no change in hot oil pressure (race dry sump pump with adjustable pressure relief valve on pressure stage ) It's so long ago I can't remember what idle pressures were, but I can never recall any concern, so they must have been OK.
 
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Oil pumps are designed to maintain a certain pressure, independent of oil viscosity. The only time a "thinner" oil will reduce pressure is if the oil is thin enough that the flow through the oiling system is so high, that the oil pump cannot pump enough oil to maintain the desired pressure - essentially exceeding the flow capacity of the oil pump. This is a rare instance, only occurring in extreme circumstances similar to an HTHS test where csT levels could get down to 2-3. If your engine sees those extremes regularly (like racing, very long idling, or very warm climates) then a thicker oil may be warranted.
 
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i have to disagree with most. Oil pumps are designed to provide a volume of oil at a specific viscosity at a certain temp at idling rpm. This insures that under hot conditions at idle, the vehicle will get adequate oil flow. It is the resistance of your engine that produces pressure. Most oil pumps are mechanical and do not regulate pressure. This is why there is a pressure relief valve in the system. I believe that there are electric pumps that can regulate pressure but most are not. In most standard vehicles you will see the pressure drop at idle from cold to hot. most standard trucks have about 40 idle when cold when hot it will drop down to 20 to 30 psi at idle. this is from the oil thinning out as it gets hotter. This will usually take about 30 mins of driving. In this case as with most vehicles on the road, you will see increased oil pressure with thicker oils. Unless you are already at a pressure that the oil is flowing thru the bypass valve which i believe is set at 45 or 50. something like that. So if you have an old engine that the oil pressure has reduced due to bearing wear or whatnot, then running a thicker oil will produce more oil pressure, unless the engine is completly toast. Keep in mind that sometimes engines are designed to have less pressure and more flow. To the point, i dont think that lower viscosity oil means low oil pressure, but in a standard 95% of vehicles on the road, it will mean lower oil pressure. In a GM 350 changing from a xw30 to 15w40 will be about 4 psi differance in hot oil pressure. It depends on exactly the temp of your engine and the exact viscosities of the oils but 4 psi is a good estimate.
 
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 Originally Posted By: tdi-rick
When swapping from a 20W-50 to 10W-30 to 0W-5 on a 1600CC engine I experienced no change in hot oil pressure (race dry sump pump with adjustable pressure relief valve on pressure stage ) It's so long ago I can't remember what idle pressures were, but I can never recall any concern, so they must have been OK.
If the 10w30 provided enough oil pressure to bypass through your adjustable pressure relief valve, it makes sense that a heavier oil wouldn't provide any additional oil pressure; it would cause more oil to be bypassed. On an older engine with more bearing clearance, heavier oil normally provides more oil pressure. The 350 in my camaro is a good example, the bearing clearances are on the "loose" side and the oil pump has an 80 psi bypass spring installed. With 10w30 the engine will produce a maximum of 55 psi of oil pressure at operating temperature. With 20w50 racing oil and the same operating conditions, it will produce 70 psi of oil pressure.
 
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Keep in mind that the oil pump's oil pressure regulation valve is a very primitive regulator...just fine for its purpose, but with a big proportional band. The higher viscosity will cause higher oil pressure unless it is leaking out of very worn bearings at a rapid rate. If the bearings are normally tight the oil pump will be pushing this thicker oil against the normal clearances resulting in higher pressure coming out of the oil pump therefore requiring the oil pressure regulating valve to open further, but the result will still be higher pressure within the parameters of this primitive regulator.
 
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Now we need a thread on viscosity vs oil pressure vs FLOW. All are important, but when is one of them too much or too little (depending on vehicle)?
 
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 Originally Posted By: bruce381
Do these two things automatically go together? - low viscosity oil and low oil pressure? all things equal viscosity will effect pressure
That has been my experience. Coupla years ago, I switched two of my diesels from 15W40 to 10W30, noting the oil pressure with a test gauge before and after the change and the pressure dropped a little with the 10W30... just a few pounds (with the oil temps approximately the same with each) both. "Low(er)" oil pressure but not "low." As has been stated earlier, it then comes down to flow. I'd be surprised if the flow didn't increase by roughly the same percentage that the pressure dropped. What I know for sure is that, the following winter, the tractor spun over lots better.
 

Fastship

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 Originally Posted By: Jim Allen
 Originally Posted By: bruce381
Do these two things automatically go together? - low viscosity oil and low oil pressure? all things equal viscosity will effect pressure
That has been my experience. Coupla years ago, I switched two of my diesels from 15W40 to 10W30, noting the oil pressure with a test gauge before and after the change and the pressure dropped a little with the 10W30... just a few pounds (with the oil temps approximately the same with each) both. "Low(er)" oil pressure but not "low." As has been stated earlier, it then comes down to flow. I'd be surprised if the flow didn't increase by roughly the same percentage that the pressure dropped. What I know for sure is that, the following winter, the tractor spun over lots better.
hmm - but how confident can you be that there is sufficient flow lubricating your engine if the only reading you have is pressure?
 
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 Originally Posted By: Fastship
 Originally Posted By: Jim Allen
 Originally Posted By: bruce381
Do these two things automatically go together? - low viscosity oil and low oil pressure? all things equal viscosity will effect pressure
That has been my experience. Coupla years ago, I switched two of my diesels from 15W40 to 10W30, noting the oil pressure with a test gauge before and after the change and the pressure dropped a little with the 10W30... just a few pounds (with the oil temps approximately the same with each) both. "Low(er)" oil pressure but not "low." As has been stated earlier, it then comes down to flow. I'd be surprised if the flow didn't increase by roughly the same percentage that the pressure dropped. What I know for sure is that, the following winter, the tractor spun over lots better.
hmm - but how confident can you be that there is sufficient flow lubricating your engine if the only reading you have is pressure?
Pressure is a resistance to flow so if there is enough pressure there is enough flow more or less.
 
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 Originally Posted By: Fastship
 Originally Posted By: Jim Allen
 Originally Posted By: bruce381
Do these two things automatically go together? - low viscosity oil and low oil pressure? all things equal viscosity will effect pressure
That has been my experience. Coupla years ago, I switched two of my diesels from 15W40 to 10W30, noting the oil pressure with a test gauge before and after the change and the pressure dropped a little with the 10W30... just a few pounds (with the oil temps approximately the same with each) both. "Low(er)" oil pressure but not "low." As has been stated earlier, it then comes down to flow. I'd be surprised if the flow didn't increase by roughly the same percentage that the pressure dropped. What I know for sure is that, the following winter, the tractor spun over lots better.
hmm - but how confident can you be that there is sufficient flow lubricating your engine if the only reading you have is pressure?
My yardsticks on that were the factory "normal" oil pressure specs for the two engines in question. Few of us have the tools to actually measure flow but the way I see it, if you have a reserve of pressure above the norm after the viscosity change, you are golden. As I said, the pressure went down just a few psi (3-4 IIRC) but was still way above the spec listed in the factory manual.
 
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 Originally Posted By: Steve S
 Originally Posted By: Fastship
 Originally Posted By: Jim Allen
 Originally Posted By: bruce381
Do these two things automatically go together? - low viscosity oil and low oil pressure? all things equal viscosity will effect pressure
That has been my experience. Coupla years ago, I switched two of my diesels from 15W40 to 10W30, noting the oil pressure with a test gauge before and after the change and the pressure dropped a little with the 10W30... just a few pounds (with the oil temps approximately the same with each) both. "Low(er)" oil pressure but not "low." As has been stated earlier, it then comes down to flow. I'd be surprised if the flow didn't increase by roughly the same percentage that the pressure dropped. What I know for sure is that, the following winter, the tractor spun over lots better.
hmm - but how confident can you be that there is sufficient flow lubricating your engine if the only reading you have is pressure?
Pressure is a resistance to flow so if there is enough pressure there is enough flow more or less.
Can you make such a blanket statement? There certainly can be cases when elevated pressure is do to lack of flow? I've seen sprayers here at work that after just being cleaned doing 90 psi, and 120 psi when plugged.
 
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 Originally Posted By: Jaymus
So, all the people that have used a thicker oil to raise pressure on old engines have just been imagining things?
No, of course not, as I said it depends on a lot of things. As Jim has pointed out above, he only saw a drop of 3psi or so. In an older engine, larger clearances (pump and bearings) the bleed off rate is higher so the pressure difference between a more viscous and less viscous fluid will be greater, but in a new or good condition engine, you will see little to no change in operating pressures. Of course without a pressure relief valve ultimate pressures will be greater with a more viscous oil, but most all engines are operating on the pressure relief valve most of the time anyway.
 
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 Originally Posted By: Fastship
Do these two things automatically go together? - low viscosity oil and low oil pressure?
It depends on the condition of the engine and what operating speed you're talking about. If the engine doesn't have excessive clearances, then it can probably reach full oil pressure on a very thin oil. The oil pressure regulator valve will lift and hold the oil pressure constant above a certain RPM. Thicker oil will only raise the pressure a tiny bit because it still causes the regulator valve to open. If the engine is REALLY worn, then a thin oil might escape from the bearing clearances faster than the oil pump can supply it so it won't reach the pressure that causes the regulator valve to open. At idle, the pump is turning slower so the difference between a thick and thin oil may be more obvious. But lower oil pressure isn't in and of itself a bad thing, if there's more oil FLOW it can be a good thing. Think of an extreme case- an oil as thick as grease even at 212 degrees F. It might build super oil pressure, but it would flow so badly that it wouldn't cool engine parts very well and wouldn't be evenly distributed throughout the engine.
 

Fastship

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thanks for all the replies - something to think about and i'll let you know what happens! my engine is an old design and an SAE20 was specified
 
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