s reg merc 3 ltr diesel ?

Hi guys,
hope someone can help me with my old luton van.
its started smoking when the engine is under load or when I rev it on idle.
The smoke is blue and smells of burnt oil.

I have checked the levels and all ok,

engine starts and idles fine and there are no unusual noises from the engine,its hasn't got a turbo,so I've ruled that one out,.any ideas on this please.

Kind regards chris.