Bonnet = manufacturer
Mechanic = not a manufacturer
Manufacturer designed the car not mechanic....if manuf says use 0w20 or 5w30, use that....in ur case since u were using 0w20, keep using it.....was there any issue before this oil change?
Pls also get ur car scanned on a computer to check if there are any faults.
Also, how was the cat converter cleaned? If the mileage is genuine then its not required to clean at such low 28k km.... ye bhi mechanic ne recommnd kia hoga, without any logic and justification.