In this tester's experiments, a single warmed up and watercooled combustion engine (green status) was tested. It was used to power a refinery with a steam engine drawing the refined fuel out to a holding tank. With a single bucket of oil in the engine feeding the refinery, it was noted that the refinery flickered between red and blue status and the refinery took approximately 8 minutes to refine one bucket of oil to one bucket of fuel. During this time, the engine consumed slightly less than one bucket of oil to complete the operation. When switching to fuel to power the combustion engine, the refinery stayed at blue status and took approximately 2 minutes per refining operation. Likewise, the engine powered the refinery long enough to produce 20 (twenty!) buckets with one bucket of fuel to power itself.
Therefore, it can be said that, for the basic operation of converting oil to fuel (not accounting for any pumps to keep tanks full, etc.) it takes approximately two buckets of oil to generate one bucket of fuel, but once fuel is acquired, it is far superior as it drives engines roughly four times as fast and each bucket of fuel contains far more energy. Given this (quick and dirty) experiment, it is clear that fuel is both worthwhile to refine as you get far more energy per bucket of raw ingredient and it also drives the engines much faster. Likewise, setting up a field expedient quarrying operation is much easier with buckets of fuel to carry to the site. It would require far fewer buckets and the operation would be self-sufficient much longer.
Additionally, it was noted that using a pair of combustion engines using fuel (and warmed up and water cooled) provided maximum indicated efficiency of the refinery with a small surplus of energy. To complete an efficient refinery operation, the surplus power can be used to power a water pump to fill a holding tank. Additionally, a pair of redstone engines can draw from the water holding tank to feed the pair of combustion engines powering the refinery. One is not enough. Likewise, using a redstone engine to pump oil from a holding tank to the refinery and another redstone engine to pump fuel from the refinery to yet another holding tank would complete an efficient refinery configuration. The only step not covered is pumping oil from the ground and into the holding tanks (which can be done separately).