A video that helps explain what and how a multi-viscosity rating is and how it is measured. The interesting part for me is the "cold" test. The difference between synthetic and other oils is not discussed on this one.
For some information on diesel rated oils this may be helpful. http://www.machinerylubrication.com/Read/28576/comparing-gasoline-diesel-engine-oils- There is a lot going on in your oil that we don't see. Concerns about catalytic converters is only a part of the issue. The temperature that is used in determining viscosity can make a huge difference. Diesel oil is rated different than gas engine oil. I for one don't know a lot about synthetic oil but the makeup in chemical composition will have some effect on mileage/fuel consumption and the contamination levels are I think the biggest differences. I suspect the idea that synthetic is more "slippery" comes from the better fuel consumption with synthetic oils. The fact of life for diesel engines making different contamination in the oil is easy to get. The idea that the oil might change emissions is less obvious. So have I helped? Or did I just open a giant can of worms?
