Mercedes Benz C Class oil use

I've suddenly hard a sharp increase in oil use. This only happens when driving at motorway speeds. We've looked at the exhaust and found oil in the intake by the turbo. So we changed the turbo, but it is still using oil. Approx one litre every 200 miles. Engine runs superbly, pulls hard and is still reasonably economical on fuel.

We did have an injector replaced a while a go and it seems that the oil consumption started after that.

We can't see any sign of a leak, underneath is clean. Have had a new cat and exhaust system fitted about a year ago. No sign of blue smoke at exhaust either. Can you help please? Thanks Pete.