ᐅ Ikea countertop sanding / oiling

Hello,
I have decided on the Numerär oak countertop. Now the question is how to oil this fine piece.
At the hardware store, I was told that the entire countertop should be oiled all around so that it expands evenly. However, I often read that only the top surface is treated.
What are your experiences? Should I install it first and then oil it, or oil it before installation? Only the top surface or all around?
Tips and tricks are welcome.
Thanks in advance.

Oil both sides (otherwise the wood will warp) – preferably using a cotton cloth. But be careful to wipe off any excess from both sides in time afterward. Then everything should work out fine!

Best regards, Expedit

I have done everything as instructed and now have a problem.
Please help quickly!

I have two boards facing each other. One is dark reddish-brown and the other is a bit lighter with a yellowish-brown tone. So they are completely different in color.

Both boards are oak and I oiled them with the same can of oil. How can this be, and what should I do now? It really doesn’t look good. 🙁
I don’t want to sand everything down again 🙁

Please help me