quality or cheap gas?

I've been using cheap gas from Costco or Superstore (grocery store in Canada) since I bought our CT a year ago. About a month ago I started having a "rotten egg" smell coming from my car on quite a regular basis for about a month straight. After googling the problem, cheap gas can give off a sulfur smell. I've switched to Shell V Power for my past 4 tanks of gas... and I haven't had the smell since. Maybe it'll come back... maybe this isn't the fix... but it seems to be doing the trick so far.

Did you use their regular, mid-grade or premium V Power? I've been using the premium V Power (15 cents more per litre here in Saskatoon, Sk. Canada compared to regular) during my fill ups. Here in Canada regular and mid-grade can have up to 10% ethanol whereas premium has none. I'll keep my nose on things for now.... no warning light has come on... but man does that sulphur smell stink. I'm just wondering how many people have this issue. Does the CT have trouble burning off the sulphur in the additives?