There are many microbes that can metabolize gasoline and petroleum. These microbes, particularly bacteria, have the ability to break down hydrocarbons, which are the main components of gasoline and petroleum, as a source of energy.
Examples of oil-eating microbes:
Alcanivorax spp.: These bacteria are known for their ability to metabolize hydrocarbons like alkanes.
Pseudomonas spp.: Many Pseudomonas species are capable of degrading various petroleum hydrocarbons.
Rhodococcus spp.: These bacteria are also known for their hydrocarbon degradation capabilities.
Marinobacter spp.: Some marine bacteria in this genus can thrive on compounds from petroleum.
Oceanospiralles: This group of bacteria can also metabolize compounds from petroleum.
How they do it:
These microbes use various metabolic pathways to break down hydrocarbons. For example, some use alkane hydroxylase enzymes to convert alkanes into alcohols, and then further break down those alcohols. Others use enzymes to degrade aromatic or resin fractions of hydrocarbons.

Now, if we can use the byproducts of these microbes, or feed the soldier on the microbes themselves, then we can take advantage of the energy in petroleum, albeit at a much lower rate than your proposed combustion model.