Yes, exactly as @nfsmostwanted wrote most of these incompetent mechanics try to hide their incompetency aka deficiency by recommending 20w50 by thinking that thicker oil will lessen engine noise plus will not leak from seals so will cover their Laziness and so called skills too
@shahshoaibh
Story is not over here, check compression test values. As your engine is new just like zero meter after complete overhaul so you can start with Suzuki Fully Synthetic 5w30 with Suzuki original oil and air filter.
BTW from where in Karachi, you repaired your Engine?