/CSB
In the UK a few years back Tesco (equivalent to maybe Coles) had a batch of dirty fuel which ruined a load of cars (literally thousands). Because it was proven that the fuel was the cause of the issue Tesco were liable, and had to pay damages to anyone who had issues related to fuel purchased from a Tesco servo.
Cue everyone who had issues with there car grabbing a receipt from Tesco's and making a claim that issues were caused by the bad fuel (how hard is it to get a mechanic to go on record to say that an issue was caused by bad fueling, knowing that he will get some work out of it).
My dad was a high up with Tesco at the time, and it cost the company millions (bearing in mind, even in recession Tesco turns over billions in profit every year)...
/end CSB
Either way, get it to a mechanic.. get it on paper that it was caused by bad fuel. Send a copy of the letter, and a copy of the receipt to the company in question. They will request samples of the fuel.. and go from there
