Spark Plug Oil Fouling

2004 R1150R w/dual spark.
Getting ready to adjust the valves for the first time (it's a new to me bike).
Have removed the spark plugs with the intent of replacing at the same time.
I noticed on the secondary plugs the threads were fouled with oil. Primary plugs look fine, though.
The bike has been riding reasonably well, no hesitation, no serious difficulties starting (usually takes two tries, does not always fire on the first).

I changed the plugs a few thousand miles ago before a long trip, and didn't notice any fouling at that time.
I also changed the oil pre-trip; currently the oil level is just a bit below max, reasonably sure it was not overfilled, does not look discolored.
Haynes says this could be oil leakage past the piston cylinders, or the valve guides.
What other symptoms might I be looking for to determine the cause of the fouling?

Newb here, thanks for any help!

A leak-down test can identify if either of those issues exists and can be used to identify which it is.

If you aren't familiar with doing one,m search YouTube for the Chris Harris leak-down test. IIRC it shows what you need to know and do.

A link to Chris Harris' leak down test on an Airhead is in this thread:

https://forums.bmwmoa.org/showthread.php?75891-On-line-Maintenance-Videos

That's a pretty decent video but I'm not sure I would condemn an engine for the leakage he has with a cold piston.

However, because you don't know the age of the plugs or have any oil consumption history, I would be inclined to slam new plugs in it and ride it like you stole it for a few thousand K and then review your oil consumption and the spark plug condition.

What's the mileage on the engine? Are you absolutely sure that it's really oil? It's fairly common that the lower (peripheral) plugs look a bit darker than the center plugs.
With Yoda agree I must:
If you are using the factory BMW plugs - Bosch - toss 'em... Get NGK plugs in there, they seem to work better for anybody who's tried them.

Center plug - NGK BKR7EKC-N (part # 2095), or BKR7-EIX (p/n 2667) if you like iridium (they do last a looooong time; just be VERY careful if you decide to reset the gap, should be 0.031")
Lower plug - NGK DCPR6E (part # 3481), or DCPR6-EIX (p/n 8196) for the iridium.
And they cost a whole lot less than Bosch (especially those from your dealer...) and your local auto parts store either has them or can get them in 2 days.

Another point: Do both plugs looks fouled? Perhaps a weak coil (below the tank, just behind the neck of the frame), but this is fairly rare (as opposed to those "stick" coils being known P.O.S.'s); also maybe the high-tension wires from that coil to the plugs could be worn or damaged by the factory's little "routing clips".