Since it all has started after when mechanic has drained out the oil, I think, best thing would be to take your car back to him, and force him to resolve the issue, without any charges.
Sometimes, garages do experiments at the cost of owners, and at the end, cheez b kharab kar daitay hain aur owner sey paisay b laitay hain as labour charges.
Owner ka double nuksa'an hota hai. Cheez b kharab ho gye, and paisay alaehda lagg gye.
