There is a definable difference between syn and non-syn. Lots of reasons.
There is, and I will say this as an opinion because the chemistry is very complex, very little difference from one brand of syn to another brand of the same viscosity and type.
Think about it ... how could it be "MUCH" superior if it is rated the same, made of the same basic ingredient, etc. Think about Mobil's high grade dino in 10-30 ... is it "MUCH" superior to Shell's 10-30 in the same circumstances?
One could safely make the point that one of the other of anything that are almost exactly the same, could be "somewhat" better or "slightly" better.
Better performance through "MUCH" superior chemistry or marketing?
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg