reduction in oil is as per design. see your car manual. i also copied the manual over here some time back.
it clearly says that reduction 1 litre of oil / 1000 KM driven is normal and no issue. just keep a check and keep the oil topped up.
it is as per toyota engine design. all vitz and corolla have this written in their manual. no need to worry. its to save the engine under strain. reduction and burning of oil is not necessarily any fault if you are not experiencing anything else like missing, loss of pick or power or any strange noise from the engine. its the design.
also manual clearly allows to use higher viscosity oil in hotter conditions. u can see in the manual.
higher viscosity oils are usually more protective for the engine. that is why u can see on the boxes of 5w-30 oils and it usually mentions fuel efficiency and higher viscosity boxes say max protection.
anyway you can discuss further if you feel like on 03335120067 with me.