Actually the necessity to change the engine oil depends on how many kilosmetres your car has run . Typically in case of petrol engines , if we use a mineral base lubricant , we should change the engine oil at around 5000 kms to 5500 kms . In case synthetic base lubricant , some where between 7000 kms to 8000 kms .
Now given any type of lubricant , mineral or synthetics , there are mainly two parts , one is the base and other is the additives . The base component remains comparatively unchanged chemically , it's the chemical nature of the additives which deplete over time . So it's best to change your engine oil , after a period of 2 yrs . But it's not necessary to change your engine oil after every 6 months .
Those service centre guys ask to change the lubricant every 6 months just to sell their stock . Simple tip ; next time when they ask , simply say that you will bring your own lubricant , and you will see they will loose all interest in changing the oil . And I say this from experience