Some have told me to wash with just water. Some have said use water and a mild soap. Others have said only use a conditioner. Others have said use silicon. Still others have told me I was dumb to buy boots with leather linings.
Leather is really the premium lining material.
It's tough, molds to your foot, can be replaced and once broken in is extremely comfortable. But, I understand your frustration at all the conflicting advice. Treat the boot linings like you'd treat your own skin, and be conservative. Rinse the boots occasionally with warm water sweat contains salt and acid that can dry and damage leather, and plain water will do a good job of removing most of that crud while leaving the leather's natural oils intact. Dry at room temperature.