Tonight I oiled my horn valves and slides. When I tried playing afterward, I could swear that the tone color was much darker and the low notes louder and easier to hit. Do you think the oiling deserves any credit for this, or was it just a lucky coincidence?

Yes, absolutely. Valves should be oiled everyday. Everyday not just three days you practice each week. And then again for every hour of actual playing time.

The oil creates a seal between the valve rotor and the valve casing. Without this seal air escapes past the valve rotor. You cant use that escaped air its wasted. Oiling the valves lets all that air be used for getting those notes and tone color.

Back to oiling; typically this my oil routine.
1) in the morning before i warm up
2) when i'm done warming up
3) before rehearsal
4) during rehearsal break
5) before teaching every other lesson
6)before an evening performance
7)at intermission of evening performance
And i might do it an 8th time if its not too late when i get home.