First of all on every used car you purchase from the market had to be tested for oil smoke test either visual oil smoke is present or not. If the smoke is not present then you can switch back to oem viscosities like 0w20
But if the smoke is present you can't switch back to oem viscosities.
Next thing is if engine is healthy then using low viscosity oils would help to increase the mileage and performance of the vehicle. High viscosities put some drag on the engine. The problem you had stated can be caused by clogged catalytic converter or th engine needs some tuneups.
As far as the oil drain interval is concerned make a customized drain interval for your vehicle.
Example
In summers if you drive in extreme jammed traffics along with full time AC then drain the engine oil on every 4000kms max and if the situation is opposite that you drive it on highways with only 10 % rush condition then you can prolong it tI'll 5000kms.
In winters you can extend your drain intervals upto
6000kms
And
7000kms
Respectively according to your driving conditions
A customized drain interval of engine oil saves your precious engine from thermal damage due to oil flashing point.
