Sorry ... not sure the engine actually knows it is happier one way or the other (mineral or syn) .... however, I would assume the benefits of synthetics are know well enough for research to make it obvious ... they are better for so many reasons ... take a tour on the search system and see what you think.
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