********* business practice Engen truck stop Kokstad

Hi we experienced ********* service from Engen truck stop in kokstad we had bought five 5l of petronas Engen oil 3 being the 600 15w40 and 2 being the 530 petronas on the 11th of February. We where meant to have bought 5 of the 600 15w40 but because of the garage not having stock our driver thought the 530 petronas would work but the mechanic suggested we don’t mix the oils so once we had finished fixing the machine by the 19th of February we sent the driver back to the garage to change the two 530 petronas (still sealed) to get two of the Petronas 600 15w40. We where surprised to find out that the garage refused to exchange the oil. Upon speaking to a lady by the name of Jasmin she had said that Engen would query and they where unable to help us. I find this very unusual and upsetting as we always use Engen oils and at times have done exchanges and I cannot understand what the issue is! I had to purchase a further two more 5litres of the correct oil and still take the oil I am unable to use. Please can someone help me understand why we are unable to exchange or get a refund on the oil?