You don't say which engine, but it's probably the rocker cover gasket that's leaking and it's pooling on top of the manifold.
 
Assuming you are talking tdi, ts not unusual for the whole air inlet system form the turbo to the cyl head to have an oil mist/residue inside it, so if you have a split ex/in manifold gasket thats where your oil will be coming from, this also applies to just about any engine with a closed crankcase breather system.
 

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