What is this pipe in my basement and leaking black grease from a connectio

good question since you can see it and we have no idea what you are asking about.

Its a vertical pipe connected to kitchen sink above..looks like black or dark brown grease leaking from a connection on the pipe...the connection looks like it is removable to possibly clean out if needed...have lived here over 30 years and is first time for this..

still don/t know anything about the pipe or what it does.

It’s probably oil from the oakum a pic would help

Based on my own experience with a vertical galvanized pipe (original 1926 plumbing) and kitchen sink above:

The pipe below the cleanout finally got occluded enough that a crust started to appear at the bottom lip of the cleanout. When I removed the cleanout plug, I could see the pipe was mostly full of scale and whatever else. The black stuff I guessed, was decomposing organic matter. Some of the scale was hard as a rock, and some was just soft goo. I put a trash can under the cleanout and used a large screwdriver to knock out what I could from the cleanout, then put the plug back in hand tight, knowing I would be back soon to replace the pipe.

Some months later, I eventually cleaned the upstairs shower which also drained thru this pipe, and knocked a hairball loose, which made its was down to the galvanized pipe before coming to rest on top of the scale. So when I ran the kitchen disposal the next time, the water and ground up food backed up into the dishwasher standpipe and made a mess under the sink. I then cut out the galvanized pipe, reamed the remaining stub that had a lead and oakum joint into a cast iron fitting in the basement floor, and replaced the galvanized with PVC. All of the pipe above the galvanized was 21st century PVC. Problem solved for now!

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Links to the State of Wisconsin Plumbing Code:
[docs.legis.wisconsin.gov]
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I am not a plumber.
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