If it were my Civic, or my fiance's, I'd probably do 5K changes anyway, especially since I have a large stash of closeout and cheap after MIR oil.
I do 5-6K on my cars with any oil.
Yeah, the Civic can go much farther, we all know that.
Still, her father is advising her based upon what he knows, and he is no doubt unaware that both oils and engines have changed over the years.
He may also be having a bit of a testosterone contest with you, since you are taking his little girl away.
Also, a couple of decades ago "synthetic" was a term of magic.
You didn't see retail shelves crammed with synthetic oils, most of which now are merely arbitrarily labeled as such based upon basestock VI.
Not a fight worth fighting, unless your wife to be shows other symptoms of excessive reliance upon her parents, and not you.