Synthetic is a "peace of mind" thing for me. Years ago I recall seeing some tests showing at what temperature conventional ATF started to break down vs synthetic, and that persuaded me that I'd be better off w/ synthetic.
One of the aircraft mags I use to get had a article about synthetic oils that said standard petroleum based oil molicules would start to break down at 325F and that the synthetic based oil molicules, back then, mid 1990s, would start to break down around 525+ F
I do remember reading about the dye in all automatic trans fluids back in the early to mid 1960 that stated that the dye would start to change colors at 275F and go from red to brown and become black at or above 325f
