I think your oil is too thick which probably could be reason for bad fuel economy, so I would recommend to switch to either 15W40 or 10W40 as a safe measure instead of directly shifting to a lower viscosity oil like 5w30 or 10w30.
Its up to you if you want to wait entire 5000km before changing oil as 20w-50 wont pose any serious harm. However, if the bad fuel economy is bothering you, than you can switch any time as well.
For 10w40 you can use Shell HX6 10w-40, or Total Quartz 10w-40, or even ZIC 10w-40.. Note that ZIC 10w-40 is Fully Synthetic whereas Shell and Total 10w40 are both semi synthetic/ synthetic technology.
I have good personal experience with Shell HX6 10w40 in Altis 1.8 2010 model, but others also should be good.
If while using 10w40 you are NOT experiencing oil burning then you can stick to 10w40 or move to 10w30. On the other hand, if while using 10w-40 you ARE experiencing oil burning then you should change to 15w-40.
For fuel economy improvement, you should also check engine air filter, Fuel filter, and also Fuel Injector. If fuel injector was never cleaned on your car it could also be a reason.