5w30 Synthetic vs 20w50 Mineral oil

But i am not getting a decent mileage out of it. 7-7.5Kmpl in city is too less as far as i know.

I don't think you'll get measureably different economy by changing oil viscosity. I didn't alternating between 0W-30 and 20W-50 (both synthetic, so going from 3.5 to 6.1 cP). In fact 20W50 might have been more economical.
Is this Suzuki SUV a 4wd or all wd? If so, getting 7.5 miles out of a liter gas is better than 30 MPG. That is pretty good for a 4wd. Even if you use the SUV in 2wd, you are still carrying all the weight around. I would keep doing what the PO did. 20w50 mineral oil on a 5000Km interval