if you want to be sure, use natural products without silicones/mineral oil, they will work fine in the armpits, too. but basically you can use any stuff you are not allergic to. next time i will mix the soda with the classic "blue can" we german women love so much: nivea creme.
don´t be afraid, just try it - conditioner, bodylotion, your favorite skin cream, whatever you like & you have at your fingertips.

Maybe you don't have enough liquid in it just add some more of your oil. I use the original recipe and just rub a small amount into each underarm. (Like I was using a cream/lotion) HTH