I don't believe that using synthetic oil gives you significantly better mileage. Any improvement, if there is in the first place, can only be around 1-3% and not more than that. I use Mobil 1 in my cars which cost Rs. 4800 and I havn't noticed any improvement in mileage at all. Still I use it only for satisfaction of mind.
Also it has been discussed before, not to use any oil, be it synthetic or mineral for more than 5,000km due to our dusty environment. Just to be on the safe side, change oil max at 5000km and change oil filter at each change