oil in exhaust in td5

I have just replaced the mid silencer section on my 1998 td5 with a straight through pipe (after breaking the other one whilst off roading)
the old exhaust had the usual and expected black carbon soot deposits at both ends of the exhaust and no evidence of any oil what so ever, nor have I had any smoke from exhaust

now I have put the straight section in one of the joints is not sealed properly on the gasket 9easy fix) but as a result I can see a drop of oil coming out of the joint, I've wiped it and it keeps appearing each time I drive it !!

any ideas where this could be coming from please ????

turbo possibly

Does it burn oil when running?

no oil seems to burn, no smoke from exhaust
if it's the turbo how do I check this ?