Thinner oil, higher oil pressure?

[argo]

I have been researching Rotella T5 10W-30 API CJ4 rated synthetic blend oil, and like all CJ4 oils, the ZDDP level is lower than when the 6.2L diesel was designed, and what the oils it was spec’d to use had. So as I have done since I started using this engine two years ago, I added one pint of STP for each gallon of oil. I have always used Rotella T3 15W-40 with STP. My oil pressure cold was 40 PSI idle, 55 PSI at 2,000 RPM, and hot oil pressure was 20 PSI at idle and 40 PSI at 2,000 RPM.

After liking what I read about this oil, and the positive feedback from Cummins and Powerstroke owners, as well as from a few Duramax owners I am friends with, I decided to give it a try. Like always, I mixed in my STP, one for each gallon jug, and filled the engine with the 10W-30 sauce. The oil has increased my cold oil pressure to 45 PSI at idle and 65 PSI at 2,000 RPM, and my hot pressures are now 30 PSI idle and 45 PSI at 2,000 RPM. I always thought that a thicker oil equaled higher oil pressures, but it is obvious that this is not always the case. I am curious how it will perform over the next 4,000 miles.

[greatwhite]

Seems fairly normal to me.

My pressure always jumps up a couple points on a fresh fill of rotella T6 and slowly works it's way back down to the 20/40 hot marks over a couple months....and that's the 5w40.

[TaxiVan]

I'm no expert, but I have noticed newer vehicles run thinner oils for a reason, and they typically get better fuel economy with it. My guess would be the thinner oil flows easier, thus possibly helping pressure to build faster by volume.

[greatwhite]

5W40 and 15w40 have the same viscosity once up to temperature.

There are many reasons for lighter viscosity oils in newer engines ranging from production tolerances to mileage concerns and everything in between.

[TaxiVan]

5W40 and 15w40 have the same viscosity once up to temperature.

There are many reasons for lighter viscosity oils in newer engines ranging from production tolerances to mileage concerns and everything in between.

I always thought the first number was the weight at temperature and the second was the weight when cold? My understanding is lower weight = thinner oil...

[greatwhite]

I always thought the first number was the weight at temperature and the second was the weight when cold? My understanding is lower weight = thinner oil...

First number is cold rating, second number is hot.

It's not intuitive.

It has to do with the way the molecules react to the heat.

It's a viscosity rating, which is not the same as "thick" and "thin":

http://www.upmpg.com/tech_articles/motoroil_viscosity/

(that's an amsoil site, I posted it for the explanations, not to promote the product)

[DmaxMaverick]

I always thought the first number was the weight at temperature and the second was the weight when cold? My understanding is lower weight = thinner oil...

W=Winter, essentially.

Multi-viscosity mineral oils are simple (minus the additive package). The first number (the W) is the oil's base weight, with a viscosity improver (VI) added that "thickens" the oil as it heats up. For example, a 15w-40 oil will be 15 weight when cold, but acts like a 40 weight when hot. These aren't the actual viscosity numbers at those temperatures, but what that weight of oil will act like, at those temperatures (if they were the actual straight viscosity oils, theoretically). True synthetic multi-viscosity oils are very different, as they don't use VI's. They are designed from the base to accommodate the temperature and viscosity changes.

[argo]

The viscosity numbers ASE assigned were arbitrary numbers to begin with. In science, all fluid comparisons are based off water. An example is specific gravity; water is the baseline, with a gravity of 1.000. A fluid that is heavier than water (in other words, would sink in water) had a specific gravity of greater than 1.000, while a fluid that would float on water (oil, for example) has a specific gravity of less than 1.000.

Given that, one might think that viscosity was based on water as well; that a viscosity of 1 would be water, and all other fluids would be based on that premise. That is simply not the case, however. They assigned an arbitrary number that has no correlation to a "constant". So an SAE 30 is not 30 times thicker than water, for example. It has been in place since the 1930s and is internationally accepted, so it can't be changed now. Keep in mind however that the 0W or 5W or 10W or 15W does not reference cold oil viscosity. No fluid can have a viscosity of 0. It is another arbitrary number that indicates (not in degrees farenheit or Celcius) how cold an oil can get before it begins to thicken and gell and cease to act like a 20, 30, or 40 viscosity, like salad oil in the freezer.

No, I am not that smart: I gleaned this information from a Valvoline engine oil educational video that we watch when covering preventive maintenance in class.