Kar Kalim

Hello to all... I am looking for some advise... I have a 07 DT 125 Rx... It's my second DT, I had one as a teenager... She's been mine since 2011, I have recently noticed that there is a lot of oily residue all over the tail end of my bike... It's been suggested "wrong oil" also suggested "wrong fuel"... As a rider not a mechanic or restorer I am at a loss... Please advise if you are more informed than I...