1998 VECTRA 2.5 V6 SRI?

Hi there, i own a cectra 2.5 v6 sri , and i am loosing a lot of oil . When driving along everything seems ok until your idling or switching engine off and then you get a bad oily kind of smell which i have found out myself to be oil sitting on the downpipe.
Also i dipped the oil level 3 days ago after topping up to max on dipstick then i checked it yesterday and its sitting in the middle on the 1,0L .
What sort of thing would cause oil to leak?