This is my observation as I travel a lot before and had filled up at different stations most of the time. I cannot say Petronas or Shell or any other brand is good. It depends on the station itself. Some shell stations, especially on the highways have good quality petrol. my car engine doesn't knock. However, whenever i filled up shell at my area, the quality is bad. I can feel the performance dropping and engine knocking just after I filled up at the station even on ron 97. I don't know how the operation of the petrol station or how they maintain their storage tanks. But what I can say from my experience is, each station is different. I would only listen to my car engine telling me which fuel it likes and use it until the engine give a bad response.