2004 Pontiac Grand Am oil change

Recently had oil change at walmart(mistake) car started and ran fine before. After that it was hard to start and blew blue smoke. My mechanic put on diagnostic and reported two cylinders missfiring, changed plugs wire and fuel filters. No help.
tried other things with no results. While checking dipstick he noticed it was overfilled, drained a quart out, left car sit and tried restarting. Car now starts and runs fine.I told this to walmart manager and he said that overfilling would not cause that problem. What do you think