@ Rahman! Which specification of oil are you buying? Shell synthetic 15W40 should be good enough. I find 4,000 kms too low for the engine to start labouring. At times, the air cleaner is clogged and not the engine oil as the oil should maintain sufficient viscosity beyond 4,000kms. Other considerations would be the age of the vehicle (aka mileage) and how often you carry out service. Under normal circumstances (Nairobi or Kenya is considered harsh driving environment), engine oil should be changed every 6 months irrespective of the mileage covered.
"You've never lived until you've almost died; for those who have fought for it, life has a flavour the protected will never know."