Fuel is a yellow, viscous liquid obtained by refining oil. When fuel is used in combustion engines, it allows the engine to run at a much higher efficiency as more energy is produced per bucket of fuel. Hence, it is strongly recommended that you refine oil into fuel.
In BC 2.2.2 SMP 7.5 times, and in BC2.2.14SSP 19 times as much energy was generated from a single bucket. In the newest development version, oil generates 3 MJ for 20000 cycles (ticks), and fuel generates 6 MJ for 100000 cycles, so fuel yields 6 times more energy overall.
When taking into account the energy spent refining each bucket of oil version 2.2.14 generated 16 times as much energy per bucket, and each bucket lasted 42.5 minutes (5 times longer than oil).