Speculation:
I understand that synthetic oils have better temperature resistance to breakdown.
I do not know if this applies to all synthetic oils but know of one engine maker, who has addressed high temperatures, in one of their models by specifying that a synthetic oil (meeting very specific standards) be used. all their other engines use semi synthetic blends.
I note that Kawasaki has recommended (not obliged) synthetic multigrade oil for use in their twin cylinder engines.
This does not mean that mineral oils are not perfectly adequate in most small motor applications.
