Dear Jayesh Patel,
Your question not is simple. Now you will understand why.
The drain interval establish only OEM but not oil manufacturer. If you used the oil with API specification not less recommended you must change oil after OEM’s recommended mileage. In this case you not loss the guarantee. But you can extend this drain interval at one's own risk. Commonly the API specification on oil package from oil manufacturer is Spanish money. They leave out of account the real condition of running. Yes, of course, we are know that API CH-4 or CI-4 oils can work long against API CF-4 oils for example. But what is in your particular case? How long - twice, thrice …? Or only ten per cent increase?
The answer to this question is Oil Analysis on regular base. The Oil Analysis gives the results with consideration all conditions - operating mode, drive manner, climate, kind of fuel, efficiency of vehicle systems (air cleaner, cooling system, fuel system) and so on. You can see the results of analysis from time to time and make the conclusion to necessary actions. For example, I use the API CH-4 (Chevron Delo 400 Multigrade) in my petrol car already 21000 Km instead recommended 10000 Km. And I see that this oil works properly until now. I have base - regular Oil Analysis.
On the other hand the main engine manufacturers make the notes in their manual, which concerns to quality of motor oils. I don’t know exactly (it does not matter), but their words are following: “if you use the motor oil meeting API CF-4 you must keeps drain interval at certain condition of running near 20000 Km; if you use API CH-4 motor oil then replacement will be after 40000 Km in the same conditions”. If that's the case you have clear conscience. You can use API CH/CI-4 longer. But if OEM don’t have similar notes then see please first paragraph.
Regards,