Don't listen to dealers here. They are buffoons and ripoffs like all over the world just to make money.
Go by your owners manual.
My pajero specifies 7500-10,000 Km intervals. I am doing them every 6000kms. Diesel filter 10k Kms, air filter 7-10k Kms with monthly/regular cleaning with compressed air.
Ensure that you regularly get your air filter checked, cleaned and replaced when necessary. Use good quality engine oil. Regularly change your diesel filter.
All of this and more information is all available in your vehicles owners manual. Please anyone who is reading this, if you don't know anything about your car and have questions, please don't rely on your local ustaad who thinks he knows what he's doing. READ YOUR OWNERS MANUAL. the engineers who designed your car from the ground up know more than local ustaad on what oil to use and what maintenance schedule to follow. If you have an imported car, I follow the 'severe' schedule for maintenance.