I don't know if an 1989 8 valve K100rs uses the same plugs but I've been using Auto-lite plugs in mine. I think they burn better and cleaner. They always look perfect when I inspect them, while the BMW plugs start to look a little off. I had a guy at an auto parts store look up the number for my bike and he ordered them for me. If you go this route, check the gap before installing.
After going around and around with dealers on this topic, I ended up using the Bosch XR7LDC plugs. Ouch $$.
According to NGK, their equivalent to this Bosch plug is the DR7EIX NGK # 5686. I might try those in the future.
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