The oil would be the next obvious thing. Proving the turbo won't be easy unless the shaft inside has an enormous amount of play.No oil on any of the joints. Engine side of intercooler and radiator covered in oil just wiped it clean to see if it returns. Variable vanes are free I can move the linkage freely. Is there anyway I can prove it’s working when it should do?
The fact you have found oil is the best clue yet.
Any leak or crack in the turbo output to engine side will cause a boost condition. I had an oil film on my intercooler a while back. It turned out to be a hairline crack on a weld. I'd be watching that for recurrence. If there's nothing else in that area that can lose oil, you may be onto an answer.