It happened as I was doing a datalog at about 5500 rpm and I never competed the log. The plugs from cylinders 1 thru 3 were fine., #4 was black and oily. I had just filled up and my tank was almost completely empty, which was the only reason I filled up at the Mobil station. (Small town with limited choices) Which I have since found out is not the same as in the USA. Mobil here is located on Superstore or Independent Grocers lots and they sell Esso gas. They are "Mobil" in name only. Like most pumps here the 87, 89 and 91 octane all come out of the same hose. 91 is ethanol free, 87 is 10% and 89 is blended inside the pump 50/50 and has 5% ethanol. I have been running this tune for 5 weeks on Chevron gas with no issues. Coincidence or gas? I suspect the fuel. Problem is I have to drive an hour one way to get Chevron. My tuner is Jason from Dizzy tunes and he has always been very conservative in the process and the car had very smooth and steady power band. My car was bought used with 8,800km (5468 miles) the car had just under 20K km (12,400 miles) when it grenaded. I unfortunately read after this happened that early production 2015's had the heads that overheated. This may have been a major contributing factor. I was also very suspicious as to why the previous owner was selling with such low mileage.....I may have found out? I have't had the heart to start the teardown yet, so that's all I have for now.