I didnt said OIL get BURNED, I said evaporation/breaking down/liquidation, in simple terms gets consumed internally as part of chemical reactions. Most important jobs of a good engine oil is to provide lubrication and reduce heat or transfer heat away from parts having frictions. During heavy load, engine oil forms an electron layer between moving parts to reduce the friction, it also takes away heat from moving parts and disposes of in other region which leads to some amount of internal evaporation, it process the sludge and removes it of which results into liquidation. A fully synthetic oil lasts long because it can maintain its viscosity for longer period of time but it doesnt mean it stays 1L from 1st km to the 10,000th KM. No oil in this world as of date does that AFAIK. Their specially developed properties allow a wider temperature range at higher and lower temperatures. With their improved viscosity index, true synthetic oils need little or no viscosity index improvers, which are the oil components most vulnerable to thermal and mechanical degradation as the oil ages, and thus they do not degrade as quickly as traditional motor oils. At times traditional motor oils burn as they dont have properties to support higher temperature range. I hope this helps, i am not sure if I am able to explain it well, please feel free to ask, i can retry. I can keep running healthy for 10,000km with regular topups of 300V, thats its speciality, but I cant do the same with traditional oils, as it would be creating lots of sludge, though the topup amount couldbe less due to heavy sludge presence.