Treat Timber Sill?

[Fortknox]

Hello! New member here
I've googled "treat timber" but haven't found anything useful.
I'm wondering if one should treat a new timber sill with some oil or something?
Haven't got the log yet but assuming it's untreated spruce? Regards

 

[Nybyggarn]

We brushed tar mixed with linseed oil on ours, don't really know if it's necessary but it was easy and didn't cost much. Smells a bit like a boat too:-