Marine Fuel Oil and Lube Oil prices

[abhansali]

Why is a marine fuel charged (cost to the owners) in metric tonnes and not in cubic meters or liters. Also, why is the Lube Oil charged in liters/cubic meters instead, unlike the fuel oil?

 

[georgeverghese]

Heating fuels have higher (or lower) heating values that are more or less constant an a mass basis than in volumetric basis. So, even when fuel gravity changes between grades or sources, the buyer pays the same if you look at the heating value of the fuel.