The engine will live longer with all castor than with all-synthetic, probably.
Not that all-synthetic is bad... a person who tunes their engine properly will get
just as long a life out of their engines on synthetic, if they never have lean runs,
and if they always use some sort of afterrun oil after each day's flying.
Castor is just more forgiving when it comes to storage, and lean runs, that's all.

Yes, you'll give up a few rpm's up top, and yes, castor makes the engines
look ugly over the years, with the brown stains... but there's definitely
nothing magical in synthetic oils that you really need for sport flying.