here's a chart, note the right hand colum, it shows original new SAE warm weight and the resulting SAE warm weight as the multi grade oil shears.

like I said, the multi-grades are great when new....but...most of them quickly shear down to a lower SAE warm weight.

 

None of the oils listed on that chart meet the API Service Category CJ-4 for use in diesel engines. In fact, at least 3 out of the 4 listed oils are for racing, rally, motorsports and motorcycles, and two out of the four are solely intended for "short distance races" . And none of the oils claim to meet any Ford Material Engineering Specifications, past or present. And the chart doesn't cite what oil temperature the cST specs were determined at.

Races are single use oil events, where not only does the oil get changed after the race, often times the entire engine gets rebuilt, by choice, or by necessity. The performance of oils formulated to excel in a short distance race of a motorbike cannot be expected to reflect the performance of an oil formulated for the diesel engine with a 5,000 to 7.500 mile oil change interval in a truck with a GCWR approaching 40,000 lbs.

Has the same or similar study been done on any oils meeting the API CJ-4 standard?

 

Actually, it is true. The term "synthetic" doesn't mean anything in particular. All majory brands do offer synthetic oils, but those oils are rarely a Class IV oil, which would be an oil made entirely in a laboratory.

Most oil labeled synthetic is a blend between Class III oil and PAO inhibitors. But the exact blend will vary from manufacturer to manufacturer, and the exact make up will vary as well.

For example, Mobile 1 products tend to have a very high PAO content level, where Pennzoil SynPower tends to be primarily Class III oil, with PAO additives.

The term "synthetic" was brought to a court case between oil companies when one was claiming their oil was synthetic, when in fact it was mostly conventional. The court ruled the term to be marketing, and not scientific.

Now days the term is generally used to identify oil that has been heavily refined and contains a strong additive package, as well as PAO to allow for that wide oil weight, but the actual use of the term is not specific to mean one thing or another.

For more info on the subject of the term "synthetic" head on over to Bobistheoilguy.com and check out the forums there.