Originally Posted By: A_Harman
Originally Posted By: pipo
Originally Posted By: dparm
The KV is not really the whole picture. It's certainly helpful information, but I would be looking at the viscosity index and cold cranking numbers.
In particular, the VI is telling of how thin it will be at startup. Remember that the VI can technically be derived from the two KV values. M1 0w40's VI of 185 is 10% higher than GC 0w30's 167. This is pretty significant.
I believe that the viscosity index is a figure that can be calculated from the 40/100 degree C viscosities, and that the viscosity index can be used together with one other viscosity figure at any particular temperature to calculate the viscosity at any other temperature. Therefore, it's not really a separate factor to consider; it reflects the viscosity spread between the 40 degree and 100 degree viscosities, and therefore correlates 100% with the viscosities.
But the higher the VI the better, as shown in this example.
I am a complete neophyte in this area, and I'm sure you're right that the kinematic viscosities are not the only factors to consider for cold weather starting protection. I'm wondering if and how the cold cranking numbers correlate with the KVs?
Since both the oils have 0W cold temperature ratings, their Cold Cranking and Cold Pumping viscosities meet the same API spec for flow at subzero temperatures. A 0W-oil must have CCS below 6200cP at -35C, and CP below 60,000cP at -40C. Unfortunately, their spec sheets don't give information to make a direct comparison. Castrol's spec sheet says GC has a Cold Cranking viscosity of 5900cP, so it comes in 5% under the limit. Mobil's spec sheet says their 0w40 has a Cold Pumping viscosity of 31,000cP, so it comes in about 50% under the limit. Looking at it this way, maybe the Mobil is better for cold flow since it is further away from the limit set by the API.
Looking at viscosity index and trying to extrapolate its effect to subzero temperature is skating on thin ice. (So to speak.) In the wide gulf of temperature between -35C, where the oil's low temperature viscosities are measured, and 40C, where the high temperature kinematic viscosity measurements start, there is no requirement for oil formulators to report viscosity. For conventional oils, the viscosity index cannot be relied on to predict viscosity much below 20C. The behavior of longer-chain hydrocarbons starting to form waxes and the behavior of pour point depressants contribute to this, although there are probably other factors.
For example, the Widman viscosity calculator predicts a kinematic viscosity of 14,800cSt at -35C, which converts to ~12,400cP dynamic viscosity. This is a much higher viscosity than the oil achieves on the Cold Cranking test: 5900cP.
I wish somebody would make a study of oil viscosity in the range of -15 to 25C. This is the temperature range where most people start their engines up after soaking down to ambient temperature. API testing is targeted toward the extremes.
Thanks for the helpful info. I won't tell your sweetheart what you said about her.
Thanks also for pointing out the Castrol data sheet approach. Why do they even bother listing the number if it's only a spec limit, especially without saying so?
Finally, I am embarrassed to admit that my C to F conversions were off, but the basic observation about M1 0w-40 being less viscous than GC at cold temps still holds.
