Rotella T 15W-40 and 10W-30, why same pour point?

[vitez]

In my recent explorations of HDEO I was surprised to find that Shell states both grades of Rotella T have a pour point of -30C°. Yet, the 15W-40 is clearly a more viscous oil with a kinematic viscosity of 120 at 40°C vs 83 for 10W-30.

Shouldn't a less viscous oil freeze at a lower temperature?

The only thing I can think of is differing amounts of VII's with very similar base stock. I understand the higher pour point of conventional oil vs synthetic, as "dino" oil contains more long chain aliphatic hydrocarbons (wax) which solidifies at at a higher temperature than short chain ones. I assume 15W-40 requires more VII's with its 5 point higher spread than 10W-30?

I'm trying to increase my edjumacation of all things tribological, so any more in-depth explanations are much appreciated.

http://www.shell.ca/en/products-services...ll-rotella.html

(click on Download the Technical Data information (PDF) link on the right hand side of the page for Shell's specs.)

 

[BobFout]

The bigger difference is pumpability and crankability at low temps.

 

[vitez]

That makes sense based on the viscosity numbers, but why doesn't a less viscous oil have a lower pour point?

 

[OVERKILL]

The pour point doesn't matter, CCS and MRV are what are relevant here with the 15w-xx labelled oil not qualifying for the 10w-xx designation.

Realistically, the 15w-xx oil has a pumpability limit of -25C, whilst the 10w-xx has one of -30C. The 15w-xx will hit the ceiling for cold cranking impact at -20C, the 10w-xx at -25C. The fact that the pour point coincides with the pumpability limit for the 10w-xx lubricant likely indicates that it is at this point that wax crystallization begins to occur significantly and the product will not pour below that point.

 

[Garak]

And you can get some 15w-40 examples with some pretty amazing pour points. That doesn't mean I'd want to use them in -40. Pour points might give some base stock hints, but don't read too much into the value.

 

[Silverado12]

I used 15W-40 up North of Edmonton, AB, but I never shut the engine off. That's the coldest I've ever been.

 

[vitez]

I wish I could just leave the engine running all winter, nothing better than a defrosted, toasty vehicle in January, but I do have to shut it off from time to time.

-25°C is a regular occurrence here in Dec/Jan/Feb, -30° to -35°C occurs less often. It was unusual to not hit -30°C this winter. I was using Amsoil AME 15W-40 prior to my rebuild. Based on what I know now (thanks everyone in BITOG!) I'm going to run conventional 10W-30 this winter as I won't have my engine broken in before, then look at all the excellent syn and semi-syn options out there.