Seems odd that a MB dealership would use something different from what is actually put in the car during production, particularly if that product is considered to be either the best or of equivalent quality to the best.
Also odd that their costs are that much different - I can't imagine there being more than a few cents involved either way ...
Finally, M1 is a known quant, V Syn may be OK ... but, certainly not in the same usage league.
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