There is oil in my 2004 BMW 320d return pipe from the intercooler to the intake manifold and smoke when started.

Simple. the trubo bearing/seal (probably more the seal) is stating to go and that is leaking oil into the return pipe. take pipe off and have a look at the turbo and see if you can see any oil leaking from inside of it. also oil in the intercooler = bad clean out after fixing the turbo = use solvent or something to dissolve oil but make sure you let the intercooler dry out before re-installing it