Go directly for 20w50 as your engine is severely detoriated don't pour in low viscosity oil and destroy the remaining engine you had left.
If in rawalpindi visit me some time so we can assess the actual condition of the engine.
Recommended engine oil for this condition is 20W50 ci-4 high detergent diesel engine oil it would secure your crank shafts and cam shafts from severe detoriation further.
You can use delo of caltex. Or any other brand drain interval is maximum 2000kms