PeachParts Mercedes-Benz Forum - View Single Post - My semi-definitive oil post

JHZR2 · #1 01-21-2004, 12:25 AM

I wrote this as a discussion from another thread, but perhaps more people will see it if I post it like this... The thread was about the difference in 15w dino vs synthetics in cold weather, and the discussion came to the point where technically, SAE specifies viscosity from a cold cranking simulator, and so theoretically, the viscosity of a dino and synthetic at some temperature point make them exactly the same.

So here is some detail about some misconceptions about viscosity spreads, cold weather performance, etc., as I have learned from numerous petroleum engineers and tribologists (sp?)

I dont intend to start an argument, or flame war. I just thought it would be interesting reading, since many like to discuss this topic, and I spent a lot of time writing it :p

there is a HUGE difference between a 15wt synthetic and a 15wt dino oil at cold temperatures. (and in disagreement to your previous post, the difference between 5200 and 7000cP at -20 is giant!) If we consider the specs for a 15wt oil, we know that the cold cranking viscosity must be a maximum of 7000cP at -20C. Similarly, we know that a 20wt is 9500 at -15and a 10wt is 7000 at -25. If it is less than 7000cP at -20, it may retest at the next coldertemperature to see if it slides under the maximum viscosity for that one as well (you had posted that this is the minimum viscosity at some temperature, which is wrong; if it was the minimum, then all oils thicker than 7000cP at -20 would be eligible for 15w- status, and so molasses would qualify for a 15wt oil, just as m1 would).

The thing is, that with a dino oil, it will be significantly thicker at 5C colder test temperatures. A synthetic fluid will remain nearly the same at 5C colder, thus the rason why they can meet a viscosity grade at say, -15, yet still pour at -54C. They may or may not qualiy for the next lower viscosity, that is a function of the oil's viscosity index.

In other words, you know that the CCS test is used to determine the low number, at the different temps shown above. However, this tests tells you nothing about how the oil is going to work when you get down BELOW the CCS temp. So a 5w-30 petroleum oil will start to thicken rapidly below -25C and have a higher pour point than a 10w-30 synthetic. For extremely cold weather, the more relevant number is the borderline pumping temp, or BPT. This defines the practical low temp limit of the lubricant - when you get down to the pour point, the oil is going to be too thick to pump out of the crankcase. The synth will pump at far lower temperatures than the dino oil, and similarly, the pumping viscosity of a dino vs a synthetic, say, 15wt at -20C will be vastly different, and in this regard, the synth oil will pump and move far better in the engine at the same temperature, as compared to the dino oil.

Now, regarding the viscosity spreads... Synthetic oils often contain NO Viscosity index improvers. Lets first talk about how a lube oil is made, as dino and synth oils are made in completely opposite methods.

In the case of a mineral oil, you start out with a base oil of 15wt(15w40 for example). You then add some VII to enhance it to the 40wt as it heats up. So you start out with a low molecular basestock and add VII's to span the temperature range of operation.

In blends, the addition of small amounts of PAO's and esters help the base stock's stability and increase the VI without the addition of high concentration of VII's.

In the case of a real full synth, for high quality true synth bases (Group IV, V), the original VI of one oil, or a mixture of various synthetic stocks with different viscosity indices will be so high that you may not need VII's. Start with a base oil of 40 weight. With the natural flow properties of full synth's (which are the reason for the benefits), you can measure using the cold cranking simulator for example, to say what a 20 weight oil would flow. If the oil will provide the same basic cP(centipoise)reading of what a 15 weight oil, then it can be classified as a 15w40, even though it doesn't use any VI improvers in this formula.

Take this same oil, 40 weight, re run the same CCS test, and now test the flow to what a 5 weight oil does, if it meets that spec, it now can be labeled as a 5w40 even though it is being used as a 15 weight in the previous example. Again, with no VI improvers. Some small lube oil companies actually use this sort of a basestock oil, so that they can improve the prices of their oils by only having one synthetic oil. It technically meets the viscosity standards for many grades, and so can be used.

The viscosity index improvers are quite important issues when considering dino oils. To make a typical 10w-30 oil:
VI Improver to Achieve 10W30 Grade
Group I 10%
Group II 5%
Group II+ 3%
Group III 1%
Group IV 0%

If you go the way of dinos, you use a particular additive package focused on the shortcomings of dino; AW, detergent/dispersants, VII's, anti-foam, etc.

For full PAO/ester synthetics, you only need mix in the additives you to beef up the AW qualities, detergent/dispersants, anti-foam, etc.

And,one can come to the conclusion that a good barrier lubrication (moly is a good example) is more important in low temp start conditions, so long as the weight of the oil is appropriate for the temp. This is regardless of the dino/synth debate, since after many hours, little to no oil will be on the engine parts, and only the antiwear package of the oil will be there to protect the parts that are subject to friction.

So this is why, technically, synthetics are still superior to dino oils. This doesnt mean at all that MB diesel engines dont last, or last any less with a dino oil. They may, depending upon conditions and oil weight, crank easier with the synthetic oil as compared to the dino oil. But, as I stated in your previous oil thread, the real reason for going with synthetics is their inherent ability to be used for extended drains, increasing their economy, as well as improved fuel economy, oil induced drag (that naturally is a sink of a few hp), etc.

I hope this proves to be informative to everyone. I am not trying to start another flame battle or anything, I am just stating the facts as I know them from my own study.

JMH