WHIZ cost Rs. 170/- for 500 ml bottle , it's enough for one time use, before oil change add this 500ml to engine oil and start the Car idle for 10~15 minutes. with intervals press accelerator paddle to rev.up. Then just drain and change filter also.
20W-50 is less synthetic, 10W-40 falls in semi-synthetic. For good oil you can drive 5000 km easily, I use to change after 6000, but after 4k~5k check oil few times randomly, if it's not reducing.