Thicker oil=less psi?

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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?
 
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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.
 
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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.
 
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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.
 
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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.
 
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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.
 
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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.
 
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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?
 
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Found your UOA: http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3329820/05_SILVERADO_4X4_5.3_14,145_MI#Post3329820 You noted you topped up with two quarts of Mobil 1 AFE 0w-30? I see viscosity at 10.96cSt, current for the ASL 5w-30 is 10.4cSt, Mobil 1 AFE 0w-30, which you topped up with twice, is 10.9cSt, so I don't see any real thickening here shrug
 
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It looks like I'm barking up the wrong tree with my oil theory. I'd still like to know why this BC 0w-30 noticeably reduced my oil pressure. Maybe my new filter has an issue? I've run several brands of syn 5w-30 in this truck in the last 100k miles mostly on Napa gold filters. The only time the oil pressure changed was with Amsoil after high miles and this new BC 0w-30 from the start, but I didn't think about the filter. With hundreds of thousands of these engines made annually I though someone would have the same experience on BITOG.
 
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Originally Posted By: UltrafanUK
...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.
Thin oil probably pours faster at ambient temps. But if its part of a positive deplacement oil pumping system that's above the min temp requirements for pumpability/pump flow (ie above -20 to 0 deg C) the thicker oil flows just as fast as the thinner oil. At operating temp of 100 deg C I'd bet you couldn't visibly tell the difference in how a 20 grade vs. 30 grade poured.
 
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Originally Posted By: UltrafanUK
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.
Got evidence of this at operating temperatures ? I assume that you are talking about things that have intentionally been given sprays to cool, not bearings (which lubricate themselves, and only require a supply to make it up, they don't get lubricated by pumping oil through them). OK, now we are talking specifically cooling sprays...then you are wrong...especially when talking the difference between say 10 and 20cst. Look at the BMW 2.3L six as an example. They are all sharp edged orifices and rely on pressure/density to control the volume of cooling flow. Drop the pressure drop the flow...20% less pressure means 10% LESS flow...to those parts "critically" requiring cooling...I've tried explaining that to the main proponent of your statement to ill effect...but hopin some will take heed.
 
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How accurate is your oil pressure gauge? The factory one in the dash doesn't appear to be a high precision instrument.
 
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