I used to use mink oil but it wasn't all that waterproof and developed mold in warm and humid weather.

I then switched to plain neetsfoot oil. Kept the water out better, lasted longer and grew less mold as long as I didn't overdo it.

After a time, on recommendation of a friend, I switched to Sno-Seal. It was even better at keeping the water out and it didn't mold for the most part. Problem was it completely sealed the leather which made the boots stifling hot and uncomfortable in hot weather and slowed the rate which the boots dried when water went over the top. It was pretty much like wearing rubber boots for about a week after a treatment. Then, over time, the leather would slowly begin to breath a bit.

With the neetsfoot oil or Sno-Seal, essentially wax, the trick was to clean the boots then to apply while warming the leather with a blow drier so it flowed into the leather. Avoid using too much.

In the end I settled on silicone. Bulk liquid in a pint tin with a dauber when I can find it and spray otherwise. After a few applications the leather becomes saturated and pretty much waterproof even if not treated for a long time. But the leather still breaths and lets heat and sweat out.