Neatsfoot oil has been around a donkey's age and is used to treat leather to keep it supple. If used as recommended, sparingly, well rubbed in, let dry a few days before being wiped dry, should not harm a leather strop. I have used it to bring back old stiff as a board strops to life. I guess if and how often you use it would depend on how stiff/dry your strop is in the first place.

Bob