I don't think your dealer has a clue! I assume your dealer's 98 RON means "Road Octane Number", not "Research Octane Number" (there is also a "Motor Octane Number). The 4.8 should be no different from the 4.4 on octane requirement. If your going to drive cross country it will have to operate on a minimum of 91 AKI (German for anti-knock index = octane) which your fuel door label should specify as my 4.4 does (in Oklahoma that’s the highest there is) and this is referring to the "Road Octane Number".

If you can't drive cross country what the hell good is the X5 to you? The knock sensors should retard the timing when they sense detonation to preclude it. I hazard a guess that your problem is either in your computer, your knock sensors or the wiring in between.

Additionally, ALL GASOLINE IS FUNGABLE. I worked for Texaco for 28 years so I know of what I speak. The petroleum companies exchange gasoline and diesel all across the country via the pipelines. If they didn't exchange the price of fuel would be so great you couldn't afford it and there would be constant supply interruptions. BP may load out of an Exxon terminal. The ONLY thing that makes a petroleum companies fuel unique is their advertising and proprietary additive which is injected at the loading rack of whatever terminal they may be pulling fuel from. The additives have nothing to do with the octane of the fuel. They are primarily to keep the vehicles fuel system and injectors clean and free of deposits. If you trade at any of the major oil company outlets you can be assured of getting a quality fuel.