In the early days of refining, straight run fuel oil was derived from the residue of the atmospheric or vacuum distillation process. Product entering the fuel oil market was of a consistent quality which meant that there were few problems. As demand for distillate products increased, refiners introduced secondary refining processes that affected the fuel oil characteristics.

In the crude oil distillation process (boiling off), there are four broad fuel product fractions generated in ascending order of boiling point temperature:

Refinery gas (primarily methane, ethane and hydrogen)
Liquefied petroleum gas (primarily propane and butane)
Gasoline and distillate fuels
Residual fuel oil

The last fraction, residual fuel is often non boiling, or at least it will not boil without resultant thermal degradation. Distillate fuels are further subdivided into several categories for specific uses again in ascending order of (overlapping) boiling point:

Kerosene, used for commercial jet turbine engine fuels, for small heaters etc
Diesel, used during cold weather conditions for automotive or truck fuels in "compression ignition" engines
Heating oil used for residential heating furnaces and also used in warmer conditions as diesel fuel for larger land-based, on and off-road engines, such as trucks, buses, earth moving and material lifting and moving equipment, farm equipment and railroad diesel locomotives. As gas oil it is used as fuel for industrial heaters and boilers
Finally, the "heaviest", or highest boiling point fractions are often blended with residual oil to make fuels for ocean going ships and large industrial steam boilers

The variation of product demand in individual countries is extremely wide and these cannot be accommodated by crude oil selection alone because of the sheer volume required. Furthermore, within any single geographical area, various crude oil sources are used and numerous refinery process configurations are employed. The result of this is that on a worldwide basis, fuel for the industrial and marine markets is subject to considerable variation in its properties. Whilst this has basically always been the case, the variations can be more pronounced than they have been in the past. Some effects of the secondary refining are:

Increased density
Increased micro-carbon residue (MCR)
Increased asphaltene (high molecular weight hydrocarbon) content
Possible increased compatibility and stability/sediment problems
Increased possibilty of contamination by catalyst fines as more cycle oil slurry is used as cutter stock

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