If your's is like most 940's, than you probably have the distributor mounted on the back of the cylinder head. If so, there are two o-rings on the distibutor shaft and body that should be replaced. There's also a seal inside the distributor that might be the problem, but it's a pain to replace and it should be apparent if this is the problem if there is oil inside the distributor itself.

You might also put RTV around the base of the distributor because the o-rings on the body are a pretty loose fit into the head and new o-rings may not cure it completely.

Because of the odd angle of the oil pan relative to the engine block, it is very possible that oil is leaking past the gasket (this isn't very likely on most vehicles). So you may need to replace it also. The replacement gaskets are segmented, so it's possible to replace this gasket without fully dropping the pan (good because it's a real pain).

The good news is none of these fixes are very expensive...I think the oil pan gasket was $7 at Autozone.