In Hongdae, go to Olive Young Hongdae Town (57 Hongik-ro 6-gil) or Hongik Univ. Station (156 Yanghwa-ro), open until 11pm, so go after dinner. The Super Melting sebum cleansing oil, 200ml plus a 200ml refill, has 2,000 reviews; at 200ml, check it in. Hongdae stays busy at night, so late hours help.