Fuel consuption rocketed after oil change

[vivalaflam]

2 weeks ago I took my 07 Avensis (1.8 engine 147hp) for an oil change. They put in castrol 5w40 fully synthetic.
But I noticed the fuel consuption rocketed.
Highway; avg. consumption was 5,8-6,2 while now after the change its between 8-8,4lt/km
City driving; avg. was 7,1-7,4 now is 10,1-10,3 lt/km
Any ideas? I havent changed my driving pace, I always shift at around 2.500rpm.
I wont be receiving my OBD2 adapter till January, so any suggestions as to what it may be are welcome.
Note that the tires have normal pressure levels.
Thanks.

Note: I dont know if the thread has to go to off topic subforum etc. If theres a place for it to be moved, please do so.
Heres my avensis for those who are wondering what the heck it is (im talking about you 'muricans )





Sent from my awesome Xperia Z1 C6903 using Tapatalk.