Engine Oil Problem

Wohnort:
New york

I have Santro Club 2004 CNG fitted, done full engine overhaul in Feb 2010 and completed 1500km running on petrol.... Car performance is good in local, not consuming engine oil... but its motorway or highway run where i have noticed that my car consumes 3,4 points out of 1-Leter [around 9 points makes 1-Leter] lubricant box after 3500Km running.... i tested Caltex 20-50 and Shell 20-50 both but problem is still there...

What may be the problem, and its rectification?

Please help.
Thanks!

I didn't find the right solution from the Internet.

References