It would be interesting to know a bit more about how efficient the transfer from the oil $ to the islamist fighters is. Tapping into the arsenals of the respective states should greatly ease the logistics and enhance the efficiency of the operations but as far as I know it left too obvious signs.

I have read little about the Syrian economy which must be in a terrible, terrible state and it is quite likely that USD, maybe somewhat intermixed with Euros has become the currency of choice in many areas.