Use Boot oil.

[TRG3]

For years, on my LL Bean pac boots, I've used the same homemade lube that I use with my traditional muzzleloaders. The lube contains deer tallow, olive oil, and bee's wax and is somewhat pliable at room temperature. A local boot shop said that this was a good leather lube. On a hot day in the summer, I smear a generous coating of the lube over the leather boot parts, turning one side of the boot to the sun until it melts and soaks in, then reversing to the other side. It only take a couple of hours for the whole process.

[Harty]

Obenaufs and hairdryer