The oil around the filler cap is proberly where some has spilt it when topping the oil up and not bother to clean it up, over time all the crap sticks to it,,
As for the linkage, how much oil are we talking?? Dripping?? The linkage is at the top of the gearbox so it's either going to be leaking down on to it,, Or the seals around the selector shafts, I doubt that tho,,
Have you replaced the clutch recently, it could be clutch fluid from being bleed up that hasn't been cleaned off,,
There's no need to worry ( that's if it's not dripping ) I'd get a can of brake cleaner and give the oily area a generous spay, brake and clutch cleaner is the best product to use on oily mess's, it breaks the oil down and just runs off with the cleaner, then evaporates after a couple of seconds
Then just keep an eye on it to see if the oily crap comes back!