4 years and counting on a set I soaked for my sons K3 750 that started out near rock hard. I soaked them in a mix of wintergreen and xylene, first try had no affect so I got each one nice and hot (not burning hot) with a heat gun then dropped them in and let soak for 24 hrs then let air dry. Pre heating the boots with the heat gun did the trick. New carb boots are still the preferred option in my own opinion but my kid was on a tight budget and the wintergreen trick did work at least for us.
And I do have a bottle of peanut oil to try on the airbox of my F2, I just haven't gotten around to it yet.