I've had the same two pair of Vasque Gore-tex boots for more than 20 years (no exaggeration). I swear by Aquaseal creme/paste (NOT their liquid version...I don't care for any liquid leather treatments). It was once also sold as the official Vasque creme/paste and liquid. That alliance is no longer. Same great stuff, though.
I trust SnoSeal to leather boots with no waterproof membrane. It's good enough for boots I know I'm going to destroy anyway. The one thing I don't like about SnoSeal (vs. Aquaseal) is that it seems like I have to apply it almost twice as often.
I've had good luck with applying my treatment and then putting the boots in the oven on a clean piece of cardboard and setting it to "warm" for 30 minutes. Normally, the leather will suck up all the treatment, but just in case it runs off the side, the cardboard is there to catch it. I stick the Gore-tex boots in the sun in a window or near a heater vent. I don't put those in the oven.
