oil on the top of engine ?

hi i own a 2005 2.5 vivaro had no problem,s until driving to london when glowplug light came on once stopped it hasn't done it again but on my return i noticed a slight loss of power at higher rev's and a trail of smoke, when i took the engine cover off the engine was awash with oil on inspection so was the underside of the van sooty black deposit's on rear doors could it be a turbo problem or a rocker gasket any help please cheers art