2006 R 500 M-Benz Oil Consumption

2006 R 500 add a liter of oil light comes on every 2500kilomters. Have has a sensor changed, a hot oil flush, and it still comes on, they tell me no leak and its withn the accpetable m-benz oil consumption ratio. Really, evey 2500k a liter of oil? What can I do, I need some help to correct this problem or at least diagnose it correctly, its costing me a lot of money with no answers or problem solved.