Marvel Mystery Oil has been around for decades. I wouldn't be opposed to using it in an old pre-emissions gasser or diesel, but honestly, with the new formulations of fuels and the resulting design changes, I wouldn't use it in anything newer. I don't know that it will hurt anything as far as seals, o-rings, etc, especially since they are all reformulated, but why risk it? The only diesel additive I would be interested in other than the ones already mentioned would be some good old fashioned sulphur, but if you have a cat in your exhaust, sulphur will ruin it pronto.
Why take the risk? There are no documented benefits from using Marvel or Bardahl, STP or any of the old formulations in new engines. Same goes with TFP teflon- that HAS been extensively studied in independent labs and found to provide the company owners vast sums of money and zero additional protection for your engine. That research is readily available online.