I strongly believe it is a simple coolant leak and is not oil.
A couple times in 7 years I found black oily mess in the same places many ST1300 owners find it, mainly in the vee between the cylinder heads under the thermostat housing, at the top front of the water pump cover, around the square and round weep holes, and running down and back the right side of the engine block. Both times I fixed the coolant leaks and the "oil leak" went away. I am talking about over 60,000 miles between "oil leaks". Not exactly a double blind test but good enough for me as empirically the oily black stuff strongly correlated to active coolant leaks.
When I removed the throttle bodies to access the alternator during its replacement 1,000 miles ago long dried oily black stuff was down in places I had been unable to access from the front and clean after fixing the prior hose clamp leaks. The top of the engine block between the cylinders is essentially flat with few raised areas, the galley plug being one, and liquid stuff can't run out and down the back of the engine because the alternator drive gear forms a dam with the cylinders and this gear drive forming three sides. Anything that spills or leaks out from the cylinder heads, coolant hoses, or engine block is contained in the vee with only the front open to draining. Now, to a reasonable inquiring mind if a engine started weeping, seeping, or leaking oil from the plug on top of the block it would continue to leak until fixed, thus the nature of oil leaks. However, I could not find any oily black stuff except what probably formed with the coolant leak that I fixed at 22,000 miles and the other leak discovered and fixed at 86,000 miles. 31,000 miles later when I was looking at the top of the engine with everything removed there was a layer of old dried oily black stuff on the flat upper surface of the block, but only where it had not been cleaned 31,000 miles prior because it was impossible to get to and clean and this includes the mystery plug and around it. If the source of the oily black stuff was an oil leak it would have continued leaking for over thirty thousand miles but obviously it did not as the stuff was dried like old oily surfaces get when caked with dust or dirt. To me, the link between coolant leaks and oily black stuff is even stronger after what I found during the alternator replacement. We know that minor leaks that make their presence known appear at first to be an oil leak. Countless threads start at this point...... It will be wet, black, and oily to the touch. It's because the water in the coolant evaporated leaving the glycol behind for road dirt and grime to stick to.