Help to diagnose potential turbo problem!

[Nick1point9]

I have a 91 RRC with the 2.5 turboD VM engine, and it routinely covers the air filter in oil. I initially thought this was coming from the crankcase breather, so fitted a catch tank. But it would seem that the oil is not coming from the breather as a) the catch tank doesn't have any oil in it and b) the induction hose from the air box to the turbo is coated in oil. The air box was thoroughly cleaned out and the only traces of oil in there go from the induction hose, through the filter and then collecting in the bottom of the air box. I checked the turbo oil feed return hose and it's perfectly clear, so I assume the turbo has had it.

Does anyone have any advice of further things I should check to confirm it's a turbo fault? And if so, where is the best place to source a turbo, or get mine rebuilt? Ebay wasn't overly successful and I don't know enough about the engine to know if other turbos are interchangeable.