regarding full synthetic oil
I initially ran Castrol 20/50 in my 70 Super Bee and found that it broke down relatively quickly.
The lifters started chattering before I thought a change was due.
In my quest to resolve that somewhat chronic issue, I tried Valvolne and it lasted MUCH longer.
That's what sold me on that brand, and I've been happy with it ever since.
I did run Motorcraft syn blend a few times when it was on sale and was pretty happy with it, too.