I switched to the 15/50W synthetic over a year ago on my 1988 XJ-S V12, I’ve done two changes so far.
First six months, oil leaks got a tiny bit worse, but then that reversed and now they are very minimal down to just one (i just have an oil pan leak). The rear main seal looks very good.
Car runs the same and sounds the same, but as Kirby says, synthetic helps clean inside. And of course can handle high temps much better.
Its expensive having to buy two five quart bottles, but i think worth it.
My personal reason to switch to synthetic, the oil molecules are identical size. So that must help coat cylinder walls properly for rings. I switched to synthetic on my 97 Volvo 14 years ago. I’ve now got 208000 miles and compression is still in the 180s across the board.
Off topic, if you have a GM TH400, they do NOT like synthetic ATF.