Re: 5W-20 or 5W-30
Although you can't separate the low and high temperature requirements when you formulate an oil, we can talk about the performance separately. Therefore, a 5W-20, a 5W-30, a 5W-40, a mineral 5W-xx or a synthetic 5W-xx will all have (within a small range) equivalent low temperature characteristics. Before anyone jumps, if a synthetic 5W-xx were "better" then it would become a 0W-xx, and would not be allowed to be called a 5W-xx anymore.
The second number designates the high temperature performance. An SAE 20 has a kinematic viscosity range of 5.6 to 9.3 cSt, an SAE 30 from 9.3 to 12.5 cSt, measured at 100C. This used to be a good enough specification, but perhaps 10 years ago a technique used to measure the oil film thickness in a running engine main bearing was used to see what the engine saw, and an additional specification was introduced called High Temperature, High Shear Viscosity. The bench measurement correlated with the oil film thickness measurement and was adopted for SAE J300.
The spec measures oil viscosity at 150C and a higher shear rate of 10^6 reciprocal seconds (more like bearing conditions.) For an SAE xxW-20, the minimum is 2.6 cP, for an SAE xxW-30 and SAE xxW-40 the minimum is 2.9 cP, and for SAE 15W-40 and higher grades, the min is 3.7 cP.
Since the requirement was put into place because it closely correlated with oil film thickness in a bearing, it would say an SAE 30 provides thicker oil films than an SAE 20.
OEMs have learned to make crank throws, bearings, cylinders, etc, "rounder" and with more precision than ever, so they don't need the thick oil films they used to in order to prevent wear. Since the lower viscosity provides better fuel economy (and because they used the 5W-20 to generate the CAFE number) they are obligated to advise consumers to use the 5W-20. There should be some comfort in taking the advice, since they would have met all the durability characteristics of the engine with the 5W-20. Except for fuel economy (and potential hassle with the dealership for warranty service) there isn't a real-world problem with the 5W-30.
It's a matter of asking "the Cobra guys" how they determined the 5W-20 wasn't providing enough wear protection over time.
