For a long time now I've been running an old London taxi as a runabout.
Not long after buying it I first heard of using two stroke oil as an additive in diesels to combat the lack of lubrication in the new 'eco' diesel fuels.
I started using it a few years ago and over a time I tried different brands,mixtures and permutations to see what difference it makes if any.
There is the chance of 'placebo effect' when you convince yourself that something has to be better because you want it to be so I used different drivers and studied reports on other web sites and forums.
The engine especially cold,is quieter,the diesel 'knock' reduced (again mainly from cold). Starting is if anything better and the engine seems smoother. MPG is slightly improved but to be strict the figures are close enough before and after that the difference is not really worth much.
I found any low ash two stroke fuel works fine. I use whatever I can find cheaply (normally Wilko or Asda) and a mixture ratio of 75 to 1 or thereabouts seems to work fine.When stopping the addition of the two stroke oil it seems to take some time and a few refills of the tank for the rattle to return fully,so maybe there is a residual effect of the oil.
The life of filters is unaffected.
The cost is pretty negligible and the positive benefits are worth it. I have now used the oil for many thousands of miles and have noticed no negative results from its use in any way.
Snake oil? Others have reported similar benefits,so probably no.
You can read about the taxi etc here.