Ryobi Edge trimmer

Well 13 years of good service from Builders Warehouse I now find I have to complain about the service at my nearest branch. I bought a lawn edge trimmer Ryobi model RPT-3000 on 6/10/2015, and 2 stroke oil and mixing bottle (Ryobi) then a few days later bought a 5 lt container for the petrol and promptly bought the petrol. Mixed the 2 together as per the instructions filled machine and did the edges of my front lawn, excellent, then went to do the back lawn and it cut out and made a strange rattling noise. So this morning I took the machine back to Builders Warehouse to be told that the oil was leaking from the machine and therefore I must have put the wrong mixture in it... Surely the leak would be due to a defective seal?? and why was it making a rattling noise...answer wrong mixture....not a loose part in motor??? They then rang the Ryobi engineer who also said I must have used the wrong mixture, and they would charge me for repairs......and please could I take it to the repair shop myself as it would be quicker......