Please explain how cold start performance varies with viscosity

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Jan 1, 2003
Consider two oils, both synthetics and both made by the same manufacturer. One is a 5w-20, the other is a 5w-30. Viscosities are 49.5 and 65.3 cSt@40°C. Viscosity indices are 156 and 161. Pumping viscosities are 22200 and 34800 cP@-35°C. All data respectively.

I'd like to know how you think these oils would perform on startup. I don't understand the significance of the difference in these viscosity and pumping values for two oils both designated as 5 weight. There is an absolute value difference of about 30% in viscosity at 40°.

How do you think this difference would affect the delivery of oil in a cold engine, with the assumption that the oil is used in a temperature environment appropriate for a 5 weight?

Please comment on how you think these two oils would perform their lubricating function at startup.

Also, would the baseline differences affect the lubrication performance as the oil and engine warmed to operating temperature?

Are there significant strengths and weaknesses of each, or is the difference between the two actually neglible, based on the data values that I reported?

Still learning, thanks much.
These are both fairly thin oils ....You'd notice better cranking with the 5w-20, but only if the temps are significantly below freezing. If your battery was marginal using the 5w-20 would be a bigger factor ....

Quite frankly, you are putting WAY too much thought into this ...most of the major petroleum oils are fairly close in performance. It's really not going to matter what you do in this case. I do think the Schaeffers Series 7000 synblends are significantly better than the off the shelf stuff, by the way.

The differences in the vast majority of Group II petroleum oils are too close to really measure with oil analysis for a given engine ....

I agree with TooSlick. At the "cold" temps that most folks start their vehicles at, you're not going to notice any difference between these two oils. It's only when you get REALLY cold (well below zero) that a 0wXX oil is going to show an advantage over a 5wXX or 10wXX oil that you can actually discern. And if all three were PAO/ester synthetics, the discernable difference may be slight indeed.
Well, I'm putting a lot of thought into it because I'm a novice yet at this. As I said, I don't appreciate the significance of the the difference. Judging from TS' reply, the difference is insignificant. That's one thing I hoped to learn. Does everyone agree?

Also, I don't know to what Group of oils these two belong.
The specs alone do not tell us enough about any oil. In this case, the specs are very close, and we know nothing about other factors including the rest of the specs, components of the additive package, and several other things, including oil analyses of other users of each of these oils.

The group of the base oil is also not enough to make a decision. We need to know all the facts, including your engine and your climate. You don't need to list your home address in your profile, but listing your state would help...unless it's a place as big as California--not much climate relationship between, say, Tahoe and Death a bit more specific.


[ January 11, 2003, 06:25 PM: Message edited by: Ken2 ]
First of all the viscosity of a 5W oil is supposed to be no greater than 3500 cP at -25 C (your viscosities have one zero too many) So these oils are equal to that value at 10 degrees less. Either of these oils would be way more than adequate for cold starting conditions.

Now the viscosity of a 30 weight is 9.3 to 12.5 cSt. The viscosity of 20 wt oil is 5.6 to 9.3 cSt. This is where the problem is for me (I'm not alone here) I'm uncomfortable with that low of viscosity. I just still feel a high quality 30 wt. synthetic or the Shaeffer Belnd is the way to go.

So essentially the cold flow properties of either of these oils is way O.K. unless you live in northern Canada or Alaska. The weakness in my mind is the viscosity of the 20 wt. is not adequate to suit me, especially during hot summer months and hard driving.

[ January 11, 2003, 06:55 PM: Message edited by: Al ]

Originally posted by Al:
First of all the viscosity of a 5W oil is supposed to be no greater than 3500 cP at -25 C (your viscosities have one zero too many) So these oils are equal to that value at 10 degrees less. Either of these oils would be way more than adequate for cold starting conditions.

Actually, that was the "old" J-300 spec limit for 5w oil. The new spec is no greater than 6600 at -30*C. See my post here about the 99 J-300 specs.

Also, don't confuse the cold cranking viscosity with the cold pumping viscosity. The J-300 cold pumping viscosity limit for a 5w oil is no greater than 60,000 at -35*C. So, these two oils are well within that limit, but I'd still like to to see the cold cranking numbers.

[ January 11, 2003, 07:49 PM: Message edited by: G-Man II ]
I'm not after any analysis as to whether these oils are suitable for where I live or how I drive or what I drive. I am simply trying to gain some perspective on the relative merits and theoretical performance of oils based upon the specs that are provided. These stats are all that are published for these two oils. This is not about 5w-thin.

I'm getting the idea that either will perform well at startup, assuming that they are used in an environment (temperature) appropriate for the weight class.
The SAE considers the Cold Cranking Simulator (ASTM 5293), and mini-rotary viscometer (ASTM 4684) tests the most important in determining cold start performance. They use both tests to assign the "w" rating to multigrade oils. The figures you quoted, 222 P for the 20-weight and 348 P for the 30-weight show that the 20-weight has better pumping performance than the 30-weight even though they are both 5w oils. The 20-weight probably beats the 30-weight in cold-cranking performance also, but you didn't provide those specs.

I think what you're having trouble seeing is how the two oils compare over broad temperature ranges, and for that you need a temp vs viscosity chart. Mobil will send you a nice one with some of their passenger car oils already plotted if you ask them for VTA018A.pdf. The chart is a real eye-opener for seeing how dramatically kinematic viscosity varies with temperature--especially cold temperature. PM me and I'll send you one.

[ January 12, 2003, 12:42 AM: Message edited by: Jay ]
Jay, can you send me this info. by email? Thanx.

YZF 150, to answer your questions re: depends what the ambient temp. is on startup. If you're starting the car at 40C (104F), then the 5-30 will be slightly thicker than the 5-20. But, since this is "hot" the flow rate doesn't matter and the oil will certainly travel to all areas of the engine (as it would if the difference was much greater). The number you posted is more applicable for comparison of "thickness" at hot 40 C (104F) and 100C (212F) because it is at these temps. that certain "film thickness" be present for metal-to-metal protection.

On the other hand, "oil flow" on startup is more pertinent at extreme cold temps.. Since both oils you refer to are a 5 weight, both will post similar numbers at a given temp. determined by SAE guidelines (I believe it's -30 - -35C or -22C - -31F). So whether you wish to compare a 5-20, 5-30 or 5-40, they will all fall within a given range determined by SAE...and hence, there will be NO significant difference in protection at startup with any of the 3 above grades or the 2 oil grades you mentioned.

There would be however, with a 0 grade. eg. a 0-40 will pump BETTER and offer BETTER EXTREME COLD protection than the 5-20 you mentioned. ie. it flows better in the -35 - 40C (-31 - -40 F)range than any of the 3 listed above.

I think a lot of the confusion here is because a certain oil "seems thinner". When? At room temp? Of course, if we now consider this...21C or 70F, the 0-40 will be the thickest followed by the 5-40, then the 5-30 and finally 5-20 because we're in the "warm" temp. range...not quite 40C (104F), but probably enough to compare relative thicknesses in our hand...but, not enough to judge relative thicknesses at EXTREME (who here has experienced -40F? I haven't) TEMPS. of cold and hot.
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