The unit of viscosity used is the Centistoke and the d6 fuel most frequently quoted are listed below in order of cost, the least expensive first : * IFO 380 â?? Intermediate d6 fuel oil with a maximum viscosity of 380 Centistokes * IFO 180 Intermediate d6 fuel oil with a maximum viscosity of 180 Centistokes * LS 380 Low-sulphur (