Part # 8121099
Siebenrock Oil Filler Funnel

On my '89 R100GS this works, but barely. The throttle & choke cables present obstacles that this part barely addresses. I didn't spill any oil on my most recent add but this isn't the ultimate solution to the challenge.

For years I've used a small plastic funnel for oil changes and top-ups, but the larger base diameter of this one allows the oil to flow much more quickly. The ridges also keep it seated perfectly in the fill tube. Genius.