Then sir why is japan making toyota 5w-30 synthetic oil ?
There is no such thing as synthetic oil can not be used in specific cars or not or specific engines , the difference between basic oil and synthetic oil is that basic oil is formulated from crude oil and has very less drain intervals like 3k to 5k kms. but the synthetic oil is not formulated from crude oil, it is formulated by chemical composition of different chemicals , which allow us to control the chemical properties of the Oil which can not be changed in crude oil.
Hence the drain intervals of synthetic oils are more than basic crude oil. Synthetic oil can go upto 8 - 15k kms and even more.