Goals from Mauricio Molina (5), Koh Myong-jin (14), Kim Jin-kyu (25pen), Palomo (63) and Dejan Damjanovic (67) saw the hosts to a one-sided victory.
The result leaves Seoul on the shoulders of Jeonbuk having played one game more, while Busan remain in sixth.
There was also a 6-0 win for Jeju United over Chunnam Dragons, with Seo Dong-hyun the star of the show for the victors with a hat-trick.
Junior Santos (18), Song Jin-hyung (33) and Jair (37) were also on target for Jeju, who climb to fourth place.
Third-placed Suwon Bluewings dropped points in the race for the title as they were held to a goalless draw at Daegu FC, while the bottom-of-the-table encounter between Daejeon Citizen and Sangju Sangmu Phoenix ended in a 2-2 draw.
Bottom side Daejeon found the target through Kevin Oris (8) and Alex Terra (22) while second-bottom Sangju tallied through Bang Dae-jong (19) and Kim Yong-tae (84).