It is far more likely you are seeing diesel fuel
rather than lub oil. Stick a bucket under the exhaust
and get a sample, smell will tell you.
Some unburned fuel
is normal for older design engines when the throttle linkage is rapidly opened, and lots of fuel is put in to bring the engine up to speed, sometime with accompanying black smoke, so you MIGHT not have a problem.
If this happens after the engine is at stable speed, then you have other problems. Maybe just overlaoding the engine with a bad prop selection, or, if this is a new situation, bad injector(s) should be considered, or poor compression
might also be an issue.