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Thicker oil=less psi? #4101363 05/19/16 08:44 PM
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RedOakRanch Offline OP
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I hear a lot about using a thicker oil to increase the oil pressure. In my 2005 Silverado 5.3 I have noticed the opposite effect. When I used to use Amsoil I always noticed my pressure drop as the oil thickened over the 15k OCI. I recently changed my oil to Castrol 0-30 which we all know is thick for a 30 grade, and sure enough my pressure has dropped, just like the thickened Amsoil. I'd say the "thick" oil drops my hot idle psi from 39 to 37 and my 2000rpm highway cruise psi from 58 down to about 52. Is this normal? Any other oil info for this engine?


1994 Ford Crown Vic 207k-Frankenbrew 0w20,30,40!
2005 Chevy 1500 Crewcab 4X4 180k-BC 0w30 $2/qt
2016 Outback 2.5 Limited 60k-shop brand?
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101388 05/19/16 09:14 PM
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Your hot idle should be about 48.

Re: Thicker oil=less psi? [Re: RedOakRanch] #4101428 05/19/16 10:17 PM
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Oil pressure is the resistance to flow. As the parts wear..the pressure will drop. So if it isn't extremely low or high I wouldn't worry to much about it unless it gets noisy.

Re: Thicker oil=less psi? [Re: RedOakRanch] #4101433 05/19/16 10:23 PM
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Papaw noticed thicker oil = less gauge pressure way back in the 70s. Everybody told him he had to run 20w50 in a Bug, but he got somewhat higher oil pressure with 30 weights, so he figured that was better.


1967 VW Beetle: Delo 15w40/ST4386
2004 Saturn Ion: Magnatec 5w30/DW5436
2014 Polaris Ranger 570: Delo 5-40/Wix 51356
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101446 05/19/16 10:56 PM
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Pontual Offline
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The problem resides in the zero doubleU. I had the same experience with them. Went to 5, 10, 15...
No noises anymore.

Re: Thicker oil=less psi? [Re: RedOakRanch] #4101448 05/19/16 10:59 PM
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restrictive oil filter.


2010 challenger R\T
2014 mercedes glk350
2003 hyundai elantra
1992 dodge w250 cummins
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101485 05/19/16 11:48 PM
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I don't think it's the filter. I always use Napa gold. I might go crazy and throw in 0w20 next to just to see what happens. I'm still intrigued why thicker oil lowers the pressure. Over the last 110k I've uses a ton of different Dino and full syn 5w30 oils. All had the exact same pressure except the 2 in the original post.


1994 Ford Crown Vic 207k-Frankenbrew 0w20,30,40!
2005 Chevy 1500 Crewcab 4X4 180k-BC 0w30 $2/qt
2016 Outback 2.5 Limited 60k-shop brand?
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101487 05/19/16 11:49 PM
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Originally Posted By: RedOakRanch
I hear a lot about using a thicker oil to increase the oil pressure. In my 2005 Silverado 5.3 I have noticed the opposite effect. When I used to use Amsoil I always noticed my pressure drop as the oil thickened over the 15k OCI. I recently changed my oil to Castrol 0-30 which we all know is thick for a 30 grade, and sure enough my pressure has dropped, just like the thickened Amsoil. I'd say the "thick" oil drops my hot idle psi from 39 to 37 and my 2000rpm highway cruise psi from 58 down to about 52. Is this normal? Any other oil info for this engine?


Oil does not normally get thicker during use. High temp shearing breaks up the molecular chains and that factor combines with some fuel contamination to thin the oil, BUT once the detergent additives are used up, the oil starts to thicken up because small lumps of mostly Carbon blowby contaminants start to build up. Many modern full synthetics are fairly resistant to high temp shearing, so if the injection system is in good order and the engine does not spend too much time at idle or short tripping, the reduction in viscosity during a normal OCI is not very great.

Another factor is the location of the oil pressure sensor unit. In most cases it is just after the oil filter, so if the oil filter starts to block it will result in a slight drop in oil pressure.

Finally, some cheap oils suffer some degree of evapouration when hot, which does result in a very small degree of thickening. That issue tends to only apply to El-cheapo dinosaur base stock oils.

Thin oil = lower oil pressure, BUT thin oil flows faster, so for engines where top end cooling is critical (Race engines in particular), using too thick an oil can be bad news.

Last edited by UltrafanUK; 05/19/16 11:57 PM.
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101488 05/19/16 11:54 PM
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RedOakRanch Offline OP
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I know there is a reason but a new filter and slightly thicker oil doesn't usually add up to lower pressure. Any 5.3 owners out there?


1994 Ford Crown Vic 207k-Frankenbrew 0w20,30,40!
2005 Chevy 1500 Crewcab 4X4 180k-BC 0w30 $2/qt
2016 Outback 2.5 Limited 60k-shop brand?
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101490 05/20/16 12:00 AM
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I saw a video that the 5.3 has had loose oil pump bolts and the gasket didn't let it leak. The housing grew with thicker oil and the build up press bleed out.

Last edited by Pontual; 05/20/16 12:01 AM.
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101538 05/20/16 03:28 AM
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I do want a new truck...


1994 Ford Crown Vic 207k-Frankenbrew 0w20,30,40!
2005 Chevy 1500 Crewcab 4X4 180k-BC 0w30 $2/qt
2016 Outback 2.5 Limited 60k-shop brand?
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101736 05/20/16 09:27 AM
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How did you determine the oil was thicker than it was new? Most oils lose viscosity as they age due to fuel dilution and shear. It is only after oxidation sets in that the viscosity starts to go back up.


2019 RAM 1500 Sport - Mobil 1 EP 0w-20, FRAM Ultra
2016 Grand Cherokee SRT - Ravenol SSL 0w-40, FRAM Ultra
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101887 05/20/16 12:02 PM
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I know the Amsoil thickened because I do UOA. The regular oils didn't change enough to matter.


1994 Ford Crown Vic 207k-Frankenbrew 0w20,30,40!
2005 Chevy 1500 Crewcab 4X4 180k-BC 0w30 $2/qt
2016 Outback 2.5 Limited 60k-shop brand?
Re: Thicker oil=less psi? [Re: RedOakRanch] #4101893 05/20/16 12:10 PM
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Originally Posted By: RedOakRanch
I know the Amsoil thickened because I do UOA. The regular oils didn't change enough to matter.


That's what I was wondering, whether you did a UOA to see if the oil oxidized or not. Can you share the UOA's?


2019 RAM 1500 Sport - Mobil 1 EP 0w-20, FRAM Ultra
2016 Grand Cherokee SRT - Ravenol SSL 0w-40, FRAM Ultra
Re: Thicker oil=less psi? [Re: RedOakRanch] #4102018 05/20/16 03:29 PM
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I did post the Amsoil one here a year or 2 ago. The others are not accessible from my phone right now.


1994 Ford Crown Vic 207k-Frankenbrew 0w20,30,40!
2005 Chevy 1500 Crewcab 4X4 180k-BC 0w30 $2/qt
2016 Outback 2.5 Limited 60k-shop brand?
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