Dharatha Priyanka Meegoda

Dear All, My car is Toyota 121, 2003. It is good running Engine, but these days fuel consumption is very low in traffic, earlier 13 - 14 done. but now it s doing only 9 -10 can you tell me the reason. and I am going to service my car next week. tell me what is the recommending oil and filters. ( My car is Manual gears.) On the other hand i wont to know what is the recommended lube oil for Diesel heavy vehicles, DS 40 or SAE 40 and why?