Air should have NO taste what so ever! If you crack the valve and sniff the air leaking from the tank, it may have a comparable smell to that of pencil shavings or no smell at all. Thats it; end of story, anything else is NOT acceptable. I once heard of a fill station located next to a bakery that delivered air that tasted like donuts if you got the air filled at a certain time of day, this turned out to be just rumor and wild imagination as no one had actually had a fill that fit this description. The LDS however, never managed to shake off this smelly label, and they eventually went out of business for other reasons.
If you have had an air fill that tastes like or smells like Oil, Rubber, Engine Exhaust, Diesel or Petroleum, Donuts, or any other possible smell and or taste. DO NOT USE THE AIR and DO NOT CONNECT YOU REGULATOR TO THE TANK FOR ANY REASON. Have the tank drained, have the tank and valve washed and inspected, redo the annual visual, and send the bill to the fill station. Above all, CALL the fill station and let them know as soon as possible. The life you save may be your buddies.