Why do turbine engines use synthetic oil instead of mineral oil?
Not being an expert, I suspect the answer is; because they can! I was told by a guy in the oil industry that with synthetic oil, the refinery pretty much reduce 'ordinary oil' to its component molecules and then put them all back the way they want them.
That way, the oil will be 'fine tuned' to do exactly what the engine and the operational regime requires.
Other than that, I haven't a clue.
Roger.