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Comment by a professional tech/mech:
"The supermarket diesel is a nightmare. One of the main reasons most people are suffering with repeated egr faults due to the fuel not burning as well and carboning up everything. My mate runs a 206 gti 180 and every 2 weeks he would have to clean his tailpipes because of soot. He asked my opinion, I blamed fuel. Turned out he was using Tescos fuel. Told him to run something good. Has never cleaned the pipes since and says it is has a lot more torque. He was amazed by how much difference it made."
Okay, that's for diesel but [< my error]. Personally I'm not inclined to trust supermarket fuel and V-Power has some healthy additives.