He meant only use name brands such as Texaco or Chevron. In my opinion your mechanic was absolutely correct.
Motor fuel is a blend of hydrocarbon molecules and oxygenates and additive packages.
During the refining process, salts, minerals and impurities are removed and helpful ingredients are added. Some companies are said to do it better than others. I use regular gasoline containing Techron and do mostly local driving. Current symptoms include hard starting when cold and pinging on acceleration. There are numerous possible causes for a vehicle failing the tailpipe emissions test.