Ikea countertop numeric / oiling

Hello, I have decided on the Numerär oak countertop. Now the question is how to oil the good piece. At Bauhaus, I was told that a countertop should be oiled all around so that it expands evenly. But now I often read that it is only done on the top. What are your experiences? Install first and then oil? Or oil first and then install. Only on top or all around? Tips and tricks are welcome. Thanks in advance

Oil both sides (otherwise the wood will warp) – preferably with a cotton cloth. But be careful to wipe off any excess from both sides in time afterwards. Then everything should work out!
Best regards, Expedit

I have now done everything like this and have a problem. Please help quickly!

I have two boards facing each other. Now one is dark reddish brown and the other is somewhat lighter in yellowish brown. So totally different colors.

I have oiled both boards in oak and both with the same can. How can this be and what do I do now, it really doesn’t look good. :( I don’t want to sand everything again :(

Please help me