Gst on reimbursement of tanker expenses

My client runs a petrol pump, he has tankers which he uses to fill petrol from Indian.Oil.Corp. site and brings to his petrol pump, therefore I.O.C. makes reimbursement of transportation.

My query is whether such reimbursement comes under perview of G.S.T. if yes than at what G.S.T. rate i have to disclose it under GSTR 3B AND GSTR 1 5% OR 18%.