The speaker begins the poem with vivid imagery, conveying just how utterly filthy the filling (or gas) station of the title is. Notice how short /i/ assonance adds intensity to these opening lines, emphasizing the speaker's shock as they exclaim:
Oh, but it is dirty!
—this little filling station,
The station is dirty, the speaker continues, because it's "oil-soaked, oil-permeated." These two descriptions mean the same thing, and the repetitiveness of the speaker's language draws attention to the culprit for the gas station's less-than-stellar appearance: motor oil.
It makes sense that a filling station would have motor oil everywhere, but the speaker's insistence suggests that there is more than a normal amount of grease in this particular spot. Still, the speaker is being a bit hyperbolic; the station certainly has oil residue all over the place, but it isn't literally drenched through and through with grease. The exaggeration here helps to convey the speaker's distaste for the scene at hand.
Asyndeton (the lack of coordinating conjunction between "oil-soaked" and "oil-permeated") speeds things up, as does the enjambment across lines 3-5 ("oil-soaked [...] translucency"). This speed creates the sense that the speaker is looking around quickly and dismissively.
According to the speaker, the place is so dirty that it's "disturbing," the whole place smothered in the translucent "black" of oil. The speaker warns someone (A fellow visitor to the station? The reader?), "Be careful with that match!" The place is liable to go up in flames, not simply dirty but dangerous.