I would say that the measured temp is going to be exactly the same in the cooling system, since it is thermostaticly controlled.
Engine temperatures are designed to remains stable for running efficiency.
There may be some oil cooled components that are running cooler, although there is no way to measure it.
It's no secret that friction creates heat and reducing it, has to make things cooler.
Will synthetic oil make a measurable differance, maybe. Will it be significant, not likely.
Most engine failures are not oil related. They are from overheating due to coolant leaks or dirt injestion, whether thru the air induction or the lubrication systems.