[neanderthal]

i run regular in my car all the time (1990 300E 2.6) and i've seen no performance detriment as a result. i used to have a misfire/ stumble when i used certain types of gas. so started paying attention to which brands/ stations caused it and avoided them. i do try to cycle in a tank of premium every once in a while

as long as you don't accelerate hard you should be ok. i really don't think there's any problem with just leaving it in there and running on it from now on. i don't profess to be all knowing however.

refill the tank with premium when you get to about halfway if the car is still running badly. the more higher octane fuel you have in the tank the higher the octane.
you do NOT have to completely drain it unless you poured in water or diesel or some other non combustible/ compatible fuel

edit. you might want to get a tuneup if the car is running badly regular. could be indicative of other issues.