Hello, I am writing this because of something I saw in "Tech talk" from the may 2017 roundel. The letter was "Use top tier, Period". I am a BMW tech with 50 years (yes 50!!!) in the auto repair business. First, Mike is right, you should always use a top tier fuel, though they are not all equal. I also agree that Shell is my favorite brand. Look for all top tier brands at toptiergas.com. The list of vendors changes, so check it periodically. Top tier is more that the "detergency" that they talk about. It is also the "additive package" which is critical to the longevity of fuel system and emissions related components. Here is where I disagree, Costco is a top tier gas. I have used it for years in my BMWs without any fuel related problems. If any vehicle check engine light (MIL) comes on THERE IS A PROBLEM! It needs to be properly diagnosed, then repaired. There are literally hundreds of faults that can turn on the light, basically anything that the vehicle programming detects indicating that emissions are 150% of the original federal test proceedure (FTP) limits. This can be in something besides the engine, for example the transmission. In response to the 535i owners question; have it diagnosed and repaired NOW! The longer you wait, the more costly the secondary damage is likely to be.