You're correct that pre-War Zeiss bellows (except on the very cheapest models from just before the War) are leather, and as such they benefit by the same care you'd give any other fine leather item (baseball glove, saddle, dress gloves, etc.).
First, if you own a bottle of Armor-All or similar spray treatment, throw it away. Those products are the worst things you can put on leather, and they're not much better for vinyl; don't use them on your car, either.
Now, get a small tube of neats-foot oil and a small block or jar of saddle soap. Clean the bellows with a damp cloth barely touched to the saddle soap (support from inside to avoid creating misfolds), then rinse with a cloth dampened with clean water; never apply more than the absolute minimum amount of water. Allow to air dry, then apply a *very thin* coat of neats foot oil, and rub into the leather until the surface no longer feels oily. It may feel slightly tacky; that's okay, it will pass in a week or so. Leave the bellows extended for about 24 hours after this treatment for the oil to cure and soak into the leather even more.
You shouldn't need to do this more than once or twice a year, unless you get the bellows wet.