Well when engine has done considerable mileage, moving parts clearances increase and engine requires overhaul. If overhauling (top or major) is not done, engine starts consuming oil at a quantifiable rate. To prevent this or to reduce this, a THICKER oil than one recommended by manufacturer is used to further drive the vehicle with less oil consumed.
This is the reason that some manufacturers recommend a thicker oil if the vehicle has done 100,000 KM as compared to a new one. Now our friend here is already using 20W50, the thickest multi grade oil available in market and is experiencing oil consumption. Hence, with thinner oil the oil consumption will increase.