Plugs work best ie spark more easily when they are operating in the correct temperature range.
The recommended plug is usually fine however if you are experiencing fouling or oiling up of the standard plug it's worth bearing in mind that the recommended spark plug heat range will have been conservatively set by BMW meaning it will be cold enough to be safe at sustained maximum speed. If you never ride at sustained maximum speed then it's probably safe to use a one grade hotter plug.
I have safely used a 1 grade hotter plug on my bike and currently use a 1 grade hotter plug on my 21 year Merc 190E because it offsets the fact that it's starting to burn a little oil at 110,000 miles and in any case I rarely use more than half it's performance.
Using a colder than standard plug makes no sense at all as it will never get hot enough unless on a modified/racing engine.
