Maruti Suzuki — First service for alto 800 lxi

Address:

New Delhi, Delhi

I bought a maruti alto 800 lxi from one of your reputed showrooms in Delhi on 27 May 2015. It has run 300 kilometers till 27 June 2015. When I called for it's first free service, I was told to get the engine oil changed or else the warranty will be void as one month has elapsed. Why is it mandatory to get the engine oil changed? Kindly guide.