Thanks all for ur replies.
The problem in my car was that its engine oil was leaking, i changed the engine seals about 2 months ago, since then oil was not leaking but its level is decreasing.
@lancer93 if i get rings fixed. does it generate other problems in engine ??
@4qi i used 10w-40 Shell yellow before , this is for first time i used 20W-50 Shell red oil. but it is decreasing very fast. about half in just 500Kms