i can confirm all turbos have oil seals, they are just a ring seal like you find on a piston, but smaller.
they are not perfect at sealing and hence why stagger gap ones are available.
however seeping without decent oil pressure is defo an issue.
you may find the old oil seal has damaged the compressor housing perhaps and hence you need an oversized one.
rebuilding a turbo is easy, just balancing needs a specialist.