0W-20 vs 0W-40

[Al]

I should know this answer...lol Are the low temp specifications the same?

 

[kschachn]

Both meet the performance requirements for a 0W winter rating.

 

[The Critic]

Both must pass the SAE J300 standards for 0W-XX in order to qualify for that rating. But keep in mind that measurement is performed at very low temperatures.

However, the kinematic viscosity at various temperature ranges can vary.

 

[BMWTurboDzl]

I should know this answer...lol Are the low temp specifications the same?

Grades exist within a range. So it's plausible that the dynamic viscosity of a 0w20 will be lower than a 0w40 at the test temp for 0w. You'll have to dig into the weeds for the oils you want to compare looking for Low Temp Cranking (cP) at -35C and the Low Temp Pumping (cP) at -40C

 

[JavierH19]

They're within the same range for pumpability. But a 0w-20 will be farther from the limit of the range compared to 0w-40 that's gonna be closer to the limit since they struggle more to meet the blend requirements. All in all 0w isn't too important until it hits at least -20f. An engine with a 5w will still start but it might make it work harder. But being in warm Texas I've shifted to low vii oils. Less polymers means less chemical shearing from fuel dilution. They typically also have a higher hths with the same kv100 compared to a higher vii oil with the same base stock, and usually a higher flash point to accompany. On top of that there's less possibility for varnish as it's the polymers that polymerize and form that varnish layer more easily than low vii oils. Though that issue can be circumvented with esters like alkylated naphthalenes. So higher quality formulations with a high vii can still be good varnish control wise but they'll still struggle more than they should in the hths and flash point categories without using more pao. Poor quality bases with lots of vii can be bad. There are syn blend 0w-20's and i can't see those being good with varnish control.

 

[Al]

So the long and short of it is that they may or not be the same BUT they both meet the same spec.

 

[JavierH19]

So the long and short of it is that they may or not be the same BUT they both meet the same spec.

They'll at least meet the same bare minimum for the pumpability requirement in the range.

 

[kschachn]

So the long and short of it is that they may or not be the same BUT they both meet the same spec.

Definitely not the same, one is a 20-grade while the other is a 40-grade. As JavierH19 notes they have similar extreme cold-weather performance.

 

[ZeeOSix]

So the long and short of it is that they may or not be the same BUT they both meet the same spec.

Both meet the same "0W" winter grade spec. As mentioned previously, you would have to compare the actual viscosity of the oil spec sheet at the 0W test temperature of -35C for cranking and -40C for pumpability between all the 0W grade oils you're interested in to see which one is actually the most viscous at those 0W grade test temperatures.