[...]The inside of the plenum is covered in a dusty/oily crud, as ive never stripped down a FI car before i have no idea if this is normal after 260,000kms. So the questions are, is this where my oil is going? There are no obvious external leaks and the car doesnt smoke.
Cougar brought up a good area to consider; a stuck PCV valve (open or closed) can cause lots of engine problems. Even if the PCV is operating normally, it can create some "interesting" conditions.
I don't know what vehicle we're discussing, but if it's losing oil and it isn't leaking out, then it must be being burned, whether that's obvious at the tailpipe or not. With the mileage you mentioned, there could be significant blowby. If so, that could be giving the PCV system lots to contend with, and in addition to exhaust gases aerosolized oil might be being fed to the intake. On many cars, temporarily disconnecting the PCV at the crankcase end (valve cover, wherever) will allow any significant blowby that may exist to be observed.
On the other hand, there may actually be a slow external leak that isn't obvious when the engine isn't running or at idle. If the PCV system (valve, hose, etc.) has gotten plugged, crankcase pressure can build up. That can force oil out of places it might not normally leak; it will usually be worse at higher RPMs, meaning that it might lose a drop every so often on the highway, but not show up in your driveway.