Charles' Law states that gases expand when they are heated. Also that gases contract when they are cooled. You can use this to suck oil down inside wood that cannot be moved by hot foods. Not my invention. Just takes advantage of a Law of physics to get the job done.
Pick the oil of your choice, I use olive oil because I have lots of it (3 liter tins) in my kitchen. I have done this with bees' wax, too.
Preheat your oven to 325F. On a cake rack, over a sheet pan, slather the wood with oil. Into the oven for 3 minutes and 30 seconds by the clock. Out it comes to cool.
You should see fizzy bubbling in the oily surface. That is heated wood air expanding. As the wood cools, the remaining wood air contracts and pulls the oil into the wood. Add more oil if you want. When cold, wipe it all off = done.
Hot food isn't hot enough to move the oil. You can't wash it out. Inside the wood, there's no interaction with the atmosphere so little to no chance of that oil ever oxidizing (going rancid.)
I carved 70 spoons and 30 forks in birch to sell in a local market. I needed a fast, simple and permanent finish to be cost effective.
NOTE: I have never carved a kuksa. I am a little concerned about the oven heat cracking the wood. Experiment on the already cracked one to see how it holds up.