Thus IFO has to undergo treatment on board the vessel to remove residues but some residues are inherent in the oil and if their percentage is too high there is a risk of damage.  For example, sulphur is present to a greater or lesser degree in all fuel oils but if there is too much sulphur,  acids are formed which will cause corrosion.  Traces of metals such as sodium and vanadium are also inherent but excessive amounts can create sticky deposits which damage the engine’ exhaust valves. In addition to the engine room equipment which deals with residues that can be filtered out, the oil must be heated in order to get it to flow easily through the injectors.  This equipment is designed to cope with oil of a certain quality and thus the most critical property of the oil is its viscosity. Viscosity can be described as the measure of an oil’s reluctance to flow freely and at one time it was common to use the Redwood Scale but today it is almost universal to classify bunker oils in Centistokes.   With all methods of referring to viscosity it is important to add the temperature on which the figure used is based and.  the viscosity temperature used when ordering bunkers is 50°C. Paradoxically, the ISO standard (see below) is based upon 100°C but this an inconvenient temperature at which to work  and testing is usually carried out at 80°C and extrapolated up to 100°C and down to 50°C respectively.  Thus 180 Centistokes (cSt) at 50°C equates to 25cSt at 100°C. Bunker oil quality is based upon International Standards Organisation (ISO) number 8217 thus when ordering bunkers one would stipulate this as the standard adding the viscosity required in cSt at 50°C.  Other standards can be used there is a British Standard (BSMA) and the International Council on Combustion Engines (CIMAC). Common Fuels are 180cSt and 380cSt, both at 50oC.