Does this darken tan-colored leather shoes?

Yes it would darken tan-colored leather shoes, but, it will not darken the shoes to the degree that mink oil or another oil-heavy conditioner would. After the initial application, the leather will be darker, but, it will lighten somewhat within a few hours (unlike oil-heavy products). A lot of the potential darkening also may depend on the leather (whether or not it is full-grain leather, has a waxed surface, is plastic-coated corrected-grain leather, etc.). However, if you're really worried about darkening the leather and you choose to use this product, it may be better to lightly apply the lotion with a cloth and wipe it off within a minute.

It says can be used on smooth and imitation leather. Confuses me, is this for GENUINE LEATHER?

Hi Aileen, Kiwi leather lotion is great for cleaning and protecting anything made of "Genuine leather" as well as "imitation leather" as long as it is the smooth or "finished" surface type such as shoes or a purse. It's not recommended for any rough surface or suede type leather though. Hope this helps!

Can this be used on oily leather Birkenstocks?

Do not use it on suede. Only use it on leather. Any leather surface should be ok. As with all things, first test it on an inconspicuous spot first. I’ve used it on leather jackets, leather boots, and car seats. Works great.