Klucel G commonly darkens leather, though usually only slightly. In every considered application it MUST be first tested in an inconspicuous location to assess that the leather's response is asthetically acceptable. Klucel G is a cellulose ether that DOES leave a film on the surface, usually not noticible except if applied in succesive coats or by using a more concentrated solution.
Many conservationists mix their own.
It seems that it will not darken, discolor or leave film. The key factor here, as with any product, is to use it for what its intended use is. This product is not a leather dressing or cleaner for supple leather but used to alleviate the problems and inconveniences of those bindings where the leather has turned to red rot and will transfer that rotted leather to anything that it touches. Essentially it is a consolidator. If you put a red rot book in a box, once it is taken out and used you are faced with the same mess.