Originally Posted by oilteq
Let me add to the confusion. A 0W-40, 5W-40, 25W-40, and all the other xW-40 oils have to fall into the same kinematic viscosity window at 100 degrees C. The W stands for winter, not weight, and the test temperature varies with the grade you are qualifying for. If my memory is right, the viscosity in the Cold Crank Simulator is taken at –20 C for a 10W-x oils. So, at temperatures above freezing, the difference between a 0W-40 and 5W-40 is insignificant.
In looking through the old posts I was impressed at the amount of knowledge, but some of it was not quite right. Group IV base oils include everything not in the other groups, which includes synthetic esters (polyol esters, monoesters, diesters, etc) and low quality naphthenic base oils no ethical person would consider for motor oils.
I have seen the literature that says “made with this or that synthetic ester.” I believe it, but I believe only a relatively small portion of the oil is synthetic ester. Esters typically have poor cold temperature flow properties, low viscosity indices, and they will attack your seals and eat the paint off of the inside of your oil pan. Well-formulated synthetic oils use 15% or so synthetic ester blended with PAO synthetic base oils.
When base oil breaks down, whether synthetic or petroleum oil, it gets thicker, not thinner. Viscosity drops when polymers or dispersant additives, but mostly polymers, shear. Polymers are also called Viscosity Index Improvers, and they are plastic dissolved into oil. Many of them are designed to shear, at least temporarily, to provide better fuel economy. I do not use them.
By the way, I do not like the current gasoline oil specifications, API SM or just that starburst symbol. A few years ago, Ford tore apart some plugged catalytic converters and found that phosphorus formed a glass like deposit on the surfaces. The industry then tried to develop a test to show an oil’s tendency to form deposits, but they failed because the test was not repeatable. After spending a few hundred thousand dollars, they said, “screw this—we will set a chemical limit of 1,200 ppm.” Later they lowered it to 1,000. In the most recent round of specifications, they lowered it to 800 ppm.
Phosphorus comes from zinc phosphates (ZDDP) used as anti-wear additives. Most cars have rolling cam followers, so it is not a big deal. Most DOHC engines, I assume including the one in the Elise, have sliding cam followers. Poke around the web and you will find there have been numerous flat tappet cam failures with SM oils.
I will not be running extended drains in my Elise or starting it at sub-zero temperatures, so if I had to choose between base oil and antiwear additives, I would not have to think it over. My contention is that if your engine does not burn oil, not much phosphorus from the oil will get to the cat anyway.
Blaine
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