The U shape is the P trap. All drains require one. They are what prevent sewer gas from entering the house. Yes, they sometimes will save a ring, but that's not their primary purpose. You can get replacements at any hardware store or plumbing shop. It is likely your drain is 1-1/2" galvanized steel. When you get the new one, pay very close attention to how it is assembled. The drain end slides into the drain pipe and is secured and sealed by a rubber gasket and a brass ring. The rest to the assembly is done in a similar fashion. These need to be only slightly more than hand tight, so don't horse on the nuts, and do not use any type of sealant. The tubes slide and swivel so there should be no problem in fitting everything together. Sometime a tube may have to be shortened, but a simple hacksaw does the job. There are also extension tubes of the rare occasions additional length is needed, but in the majority of the cases, the standard set will be all you need. The black goo is pretty normal.