I second this, if you use a very small bead (which is all you should need if the wood is well cut) then it'll be basically invisibleI believe clear sealant is the norm.
I would be concerned by this as any water on the work surface will find it’s way between the two and rot them.I didn't use anything, I found both bits of wood very flat and there isn't really a gap in mine at all, certainly nothing worth filling.
Where you'll get gaps is between the wall and the upstand. Those I filled with some acrylic sealant and painted with the wall colour and they look pretty seamless.
Putting a new kitchen in and we will have oak worktops with oak upstands, planning to treat both with Osmo oil, but what would I use to seal the gap between the upstand and worktop to stop debris and stuff getting in?