is it a fuel injector problem????

I have a 2005 g35 that i bought in september and i live in Montreal Canada... for some reason, I have had the check engine light ON for a while, and when I checked it... I was told I needed to change the injectors which cost 230$ a piece at the dealers...

Now the issue is, every time I refuel at Petro Canada 94 octane or 91 octane the check engine light goes away... but when I use Shell V-power I got the check engine light after emptying a quarter of a tank...

I heard that Shell 91 octane is the best... but my car complains from it.... anybody has any explanation to why this could be happening???

one last thing... I heard that petro canada add 10% ethanol to their gasoline... is this true???