from what i know

wether the oil evaporates or not is depending on the base oil used to manufacture them. low quality base oil makes lousy lubricant oil, like those what we call "kancil" oil. The flash point temperature of the oil is low, therefore if put into big/hot engines, they will evaporate quickly, coz the oil breaks down. Branded manufacturer's use good base oil, thus their oil can withstand high temp. U'll need to search for oil specification if u really want to know which oil can last better in high temp.

wether the oil evaporates or not, is hard to tell. Coz if your engine is lousy engine, where piston ring tolerance is large, the thin oil will sip through and burn off in your engine too. thats y ppl recommend 20w-50 for old engines.