I agree Steve, it is a good idea to only use the recommended octane.  My Grammy swears by Shell premium or plus - whatever the highest is - for her crappy station wagon that is over 20 years old.  She absolutely loves it and won't give it up and it still runs like a champ. 

I too have had to use the additives to correct the build up but I can't remember what - I think STP of somesort and that was back when I had my Fiero (no it never caught on fire and was the best car I ever had).  It never hurts to use a cleaner additive every now and again.