The oil drain out of the turbo should be checked for any restriction. A buildup of crankcase pressure caused by a bad PCV valve or excessive blow-by can also cause the turbo to leak oil.
Oil leaking on the intake side of the turbo is strong evidence of an air restriction.
If the air cleaners are working properly, inspect the intake hoses for soft spots, they may be collapsing. This could also cause occasional black smoke and low power. If an engine is idled for an extended period of time it is not unusual for the turbo to leak a little oil. This leak should go away once the engine is loaded up and boost is generated.