Heating problem

Can anybody help I have a 2013 Audi A5 2.0 TDI cabriolet and about a month ago found in my water coolant loads of oil it turned out to be oil cooler which was changed but mechanic said that there will always be oil in the system as its impossible to really get rid he tried hosing through as much as possible anyway a month on and my car is now blowing cold air when idling on motorway or over 2 thousand revs it warms but goes cold when back in neutral I tried flushing as much water out the coolant myself with tap water to get rid of some oil residual which I now no is bad as should use distilled water or proper coolant but sorry im a proper novice when it comes to cars I'm guessing the mechanic hasn't done a great job in probably flushing the system and just used washing up liquid and water and as a result the residue of oil build up and water stones has probably blocked up the system any ideas thanks