>>18488460 (OP)After they dry off after getting wet. Depends how often that happens. If they never get wet and it's dry and dusty all the time, you should brush them clean and oil them once or twice a year. If the leather is pilling or cracking it's way way too dry.
Dubbin is synthetic/petroleum based and mink oil is natural. I prefer mink oil because it seems to 'soak in' better and not leave a greasy finish that you need to buff off, like with dubbin.