Oil pressure sender
Just to add my experience:
My '05 950 Adventure started leaking oil, a little at first, but gradually getting worse. It looked like it was coming from the base gasket of the rear cylinder, but after reading several posts about the oil pressure sender between the cylinders being a likely culprit I stripped the bike down. Although there appeared to be little oil around the sender, it was the highest point where there was any oil. After cleaning everything and replacing the sender unit and the copper crush washer with new items the bike was rebuilt. Result, no further leaks.
The part is less than £7, but you're looking at a good days work to strip and rebuild. I found the fuel lines in the airbox a particular bitch to get back onto the carbs. However, after initially thinking I had a potentially huge bill for an engine rebuild coming, it was a relief to cure the problem with very little expense.