Oil-based scents are derived from the highly purified oily fractions of processed natural foods like herring, shad, and crayfish, primarily composed of fatty compounds renowned for their insolubility in water. As such, these oily substances offer bass little if any chemosensory information to work with. Yet they are often touted as fish attractants, which they definitely are not.
Even when small droplets are released from an oily attractant, the droplets merely rise (oil being less dense than water) and spread out across the surface, still providing the bass with no chemosensory information. Oil-based scents never have been and never will be true attractants —and that's just a physical reality.
Oil-based scents do make great masking agents for covering repulsive odors and tastes. Masking agents work in two ways. One is to dampen offensive odors and tastes by mixing in positive substances. The positive agent confounds the chemosensory system, diluting the offensive nature of the repellent. Aerosol air fresheners of the type used in cars and restrooms are based on this masking strategy. In the other strategy, the masking agent actually limits physical contact with the offensive agent. Oil-based scents overlay the molecules of a fish repellent with an impermeable barrier, thus preventing the repellent from dissolving into the water and reaching a bass's chemoreceptors.
While oils are superior masking agents, we must keep in mind that they are exceptionally poor flavor carriers. The very properties that allow an oily scent to prevent offensive agents from reaching the chemoreceptors also prevent the receptors from contacting anything positive. A simple Betty Crocker-type experiment should bring this reality home. The next time you're in the kitchen, put a pinch of sugar in your mouth; if your taste system is normal, you'll taste an overpowering surge of sweetness for several seconds. Rinse your mouth out with water, wait a minute or so, and then do the experiment again, only this time swish some cooking oil around inside your mouth before you eat the sugar. You should still be able to taste it but much less intensely. In fact, chances are everything will taste pretty bland until the oil wears away.
Using oil-covered baits to lure a bass creates the same dilemma. Grabbing them may spare the bass contact with offensive flavors, but the coating effect of the oil also blocks the taste receptors from contacting any positive flavors. There's no way to uncouple these two effects: you simply can't have one without the other.