6 months is fine- its not like that it's exposed to sunlight and humidity etc. It's in your engine, sealed! A pack of engine oil has a shelf life of a year. Once opened, 6 months.
But, to be on the safe side, get it changed every 3-4 months, your car is worth more. If you change oil every 3 months and filter + oil cost 3000-/, that's 33.33 rupees per day.
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