Heavy fuel oil (HFO) 380 centiStoke (cSt) has long been the bunker fuel of choice for the overwhelming majority of shippers burning residual fuel. At the world’s biggest bunkering port, Singapore, HFO 380 cSt accounts for three out of every four metric tonnes (mt) of marine fuel sold. However, in the last eight years, sales of the lower viscosity HFO 180 cSt have fallen sharply in contrast to the more viscous HFO 500 cSt, which have grown exponentially. In 2009, the latter accounted for 10.1% of the 36.4 million mt of marine fuel sold at the global bunker hub that year. Fast forward to 2017 and more than one tonne out of every five tonnes of marine fuel sold at Singapore is 500 cSt.