Psi dropping as oil gets older?

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When I put fresh oil in my truck on cold start my oil pressure will read 62 psi at peak for the first few months. At the end of my oils life it will read 57psi at peak on cold start. Is that just due to it thinning out due to is being older and used?
 
If all things are equal, it's likely thinning over the course of the oci but loose rings or internal leaks like a valve stem seal can effect oil pressure too.
 
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Are you comparing the startup pressure at the same outside temperature? A few degrees of difference would make a difference in viscosity, and therefore pressure.
 
It helps to have a diagram of your oiling system . Here is a Chevy LS V-8 engine. The oil pressure sensor is at the top right of the photo. The oil pump is at the front of the engine (bottom left). The oil filter is at the rear of the engine. (bottom right) . After the oil passes the oil pressure gauge it still has to travel down to the main bearings, camshaft bearings, lifters and rocker arms. If your oil is shearing down, yes the oil pressure indicated on the gauge will be lower because the thinner oil is making past the bearings with less back-pressure.
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Oiling system LS engine.PNG
 
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Fuel dilution (most typical) and shearing of VII's will result in some viscosity loss over the OCI, which can be observed with a reduction in oil pressure. Perfectly normal.
 
oil shear - will drop psi way before fuel dilution could - and if it was fuel dilution it would just keep going....oil shears a lot at first and then 'holds on' while slowly shearing more.
 
wait long enough and oxidation will eventually increase the psi back to normal
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Originally Posted by tundraotto
oil shear - will drop psi way before fuel dilution could - and if it was fuel dilution it would just keep going....oil shears a lot at first and then 'holds on' while slowly shearing more.


Nope, fuel dilution i the primary driver of viscosity loss, which can be readily observed with a drop in flash point via a UOA. There is a percentage of it that flashes off once the engine is run at temperature for a while. This was mentioned a while back by an employee at Castrol who indicated that most modern VII's are quite shear stable and that in their testing, fuel dilution was the biggest contributor to viscosity loss, despite most on here blaming shear.
 
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