D-4 7U oil in exhaust

There are fellows with more knowledge than me that will better help you, but are you sure it is oil and not fuel or even water that is spraying out. Wet soot appears to be oil. I know on my 7E if I haven't covered the pup exhaust and we get a rain the next time I start it the underside of the canopy looks like hell. You may have already checked this out. Thought I would mention it in case you hadn't.
Have a good day, Garry

Its called lots of names "slobbering" is typically what its called. But the bottom line is its unburnt fuel from idling too much. Think about it, engines cold maybe the Injectors are a little weak. You start it up, the Thermostat stays shut to warm it up as you indicated it clears up in about 5 minutes, meaning when the engine warms up.
The cure?
Simplest is to take it out and work it steady and hard for at least 2 hours (4 to 6 preferable).
If you want to throw money at it, then get some work done on the injection system.