Sounds similar to the leak on the s40 my brother had (ex Spac). Replaced the turbo return seals, didnt fix it; thought it could be the cam seal as suggested here but it was pretty dry.
We concluded that it had to be the rear main seal, and oil was being thrown up onto the back of the engine by the driveshaft. (This was a manual, not sure if it could be the same with an auto.)
Interestingly it was all completely dry until we did an oil change, then about a week later it started leaking badly. Sounds like your experience may be similar?