I feel like we are deeply programmed on an instinctual level to avoid having shit in our mouths. I always smell an asshole carefully before I open my mouth, sometime I even penetrate it knuckle-deep with a finger and discreetly smell my own finger as an inspection, while I have my partner distracted with something else so it doesn't feel too clinical, and I continue to re-inspect the environment discreetly as I consider venturing deeper with my tongue. I've sometimes noticed once I get my face down there, that things are not as clean as the guy claims, because some guys THINK they know how to clean their assholes and they DON'T, but I can always tell IMMEDIATELY with my nose since it's such a deeply instinctual aversion that's programmed into us. Shit stinks if it's present and it's NOTICEABLE! So it's just a matter of being straightforward enough to tell a gentleman that he is unfortunately not clean enough for me to stick my tongue down there (unless he never asked for it in the first place, then I simply play it off as taint-licking and work my way back up to his cock). If the guy really wants to be rimmed and just isn't clean enough, it's time for a mutual shower so I can freshen things up myself with plenty of soap, hot water, and my wiggly little finger! It doesn't take much to make an asshole yummy and fresh. I haven't tried putting oil on afterward, but I keep almond oil around for my hair, so I'll have to try that! Good suggestion!
Assholes do have their own unique smell, but they should never ever smell like shit if they're actually clean. If you smell it, it's there, and your mouth doesn't need to go there in that case. I feel pretty good about my ability to tell the difference between a nice clean asshole and a dirty one! They smell very different! A clean rosebud smells like pheromones and sweat, not like shit. I'm sure others who like to eat ass can attest to this difference.
Having a healthy diet full of fibers always helps too, as with any kind of assplay.