Oil change

S'cuder said
Sorry, but with all due respect, that's incorrect.

The first number that is the prefix for the "W" is the grade assigned to the oil based on it's flow properties at 0°C, or 32°F, and the second number represents the flow properties at 100°C or 210°F. Or, in other words, the freezing and boiling points of water.

A 15W oil, regardless of it's base stock, will flow in the range specified for it at 0°C, and not out of it. Doing so would mean it is not a 15W, but rather the oil would be assigned the grade number that is defined as that which flows as it actually did.

Thanks for the respect LOL, but please read the below link and you will see where the temperatures are set for viscosity testing of engine oils: 40C and 100C. What is getting confused here is the testing specs versus the SAE and ISO published specs for very low temperature grading. Those are 2 different things. And I am not aware of any testing or grading specs at 0 degrees C for engine oils.

Do as suggested and try pouring a full synthetic vs a full dino oil with the same low temp rating when chilled in your freezer; you'll see the difference right off. And look at the cold pour point temps on a full sysnthetic vs full dino... the synthetics will consistently have lower pour point temps.

S'cuder said
Sorry, but with all due respect, that's incorrect.

The first number that is the prefix for the "W" is the grade assigned to the oil based on it's flow properties at 0°C, or 32°F, and the second number represents the flow properties at 100°C or 210°F. Or, in other words, the freezing and boiling points of water.

A 15W oil, regardless of it's base stock, will flow in the range specified for it at 0°C, and not out of it. Doing so would mean it is not a 15W, but rather the oil would be assigned the grade number that is defined as that which flows as it actually did.

Synthetics generally have a higher viscosity index than dino base oils, which means they are more resistant to thinning when heated. That means that less additive, such as viscosity index improver or pour point depressant, can be used to make it perform and stay in spec. When additives are consumed, that's a way we get to the situation Castrol likes to describe as "breakdown."

Your assessment of how ZDDP works, however, is spot on as I understand it.

Thanks for the explanation. I will look into it more and see where I am going wrong on the low temperature testing. I am thinking that the 40/100c testing is for checking viscosity index and that is where I am going off the rails with this. And yes with the lower VI's added to synthetics; that is also as I understand it.

I will say that I have not ever read of engine oil viscosity testing at 0* C. I do see testing of multi-grade engine oils at the very low end of the temp range but those temps are well into the minus tens of degrees C. (That test is for 'Maximum low temperature cranking and pumping viscosities'). Like here: SAE Viscosity Grades – viscosity table and viscosity chart :: Anton Paar Wiki

Assuming that is correct that those tests are done at those much lower temperatures that 0*C, then the viscosity behavior at temps like 0*C are undefined, and that is what I was thinking of as 'cold temp operation'; not many guys posting here are starting their hot rod engines at -30C LOL. If you do as suggested and put a full synthetic and a full dino, both of the same grading, in the freezer at say 25F, and then pour them out, the viscosity difference will be obvious. This difference (syn thinner than dino) will also be seen at normal room temps. The viscosity index difference between the 2 types extends well down in the temperature range, not just from 40C to 100C.