if i got this right the economy will get worse after an oil change

because i have heard that old oil gives you better mpg as its more likely to have decomposed or whatever you would call it

but at least the new oil is better for the engine

also if they put lower quality oil in your engine than the factory fill which could have been better anyways

hopefully this is helpfull

i also noticed a drop on my mpg and tried looking into it yonks ago and thats what i sorta found

if i got this right the economy will get worse after an oil change because i have heard that old oil gives you better mpg as its more likely to have decomposed or whatever you would call it

Logic would indicate to me that new oil should offer better lubrication coupled with lower friction, giving better mpg. However I'm happy to be proved wrong. Oilman has posted elsewhere that Fuchs provide the factory fill oil. Assuming that this is Fuchs Titan Supersyn SL PD SAE 5W-40 - the only 505 01 oil they produce - then it should provide better lubrication, life and mpg than most other 505 01 oils since it is fully synthetic. BP Special Visco V 5W-40 is also fully synthetic.

DGW i think it's something to do with the oil becoming more gungey when old so there is not as much oil rising up when cold the 5w-40 changes soo no oil when on start up becasue it can't get there quick enough but when warmed up it will run better MPG because its thicker less friction but really bad for the car when cold

i maybe totally wrong here

Perhaps they tightened the handbrake during the service - the rear brakes could be dragging slightly, reducing your mpg?