Most people know that when you add oil to a fire, it grows rapidly and becomes more powerful. So, why isn't added into the propellant just before it comes out of the engine?
Because 'oil' is basically what most rockets already use for fuel.
To be a useful fuel you want a high energy density so your rocket is not lifting 'stuff' that does not burn, ideally both in terms of volume and mass (light fuel weight AND low tank weight). In the middle of this table from the Wikipedia page
you see most of the things we think of as 'oil' in the middle with a trend for less useful densities going for longer (oiler?) chains with Kerosene, pretty close to the canonical rocket fuel as RP1 bang in the middle.
Natural Gas and Hydrogen are 'better' but depend on very good tank design to not end up with big and/or massive tanks eating the performance advantage.
Oil, whether vegetable or petroleum based, does not burn nearly as cleanly as other types of fuel used for rockets. One of the types of rocket fuel are those based on hydrazine and its derivatives; these types of fuel do not contain any carbon chains, and thus easily and cleanly burn completely to volatile products. Meanwhile, oil is composed of compounds which contain relatively long carbon chains (about 12 atoms). In circumstances where such fuel cannot be burnt completely, like during the engine shutdown, it could polymerize or carbonize, leaving non-volatile deposits which build up in the engine and the nozzle, and rockets are extremely sensitive to such contamination.