This was before my AM-coffee, misplaced the question mark.
As you might expect, it depends on the conditioner.
Currently, I like Bick4. Aside from a thin, dry, almost paraffin-like (?) residue that results from excess being left on the leather, I don't see how you could over-condition with it. And in normal use, it never darkens the leather...any colour leather.
But I said "currently," because these products change. A respected member of SF recently told me that Lexol had recently decided to add more water to its formula. Sometimes such changes are as a result of EPA regs., sometimes just economies of scale.
I don't know. I'm not a chemist...I'm just going on my experience with lots of different products and leathers and environmental conditions over the years.
Naturally, any conditioner with heavy un-homgenozed oils or fats will leave a residue in/on the leather...and in the leather. And yes, it is possible to so overwhelm the fiber mat with oil that it becomes a rag. I've seen logger's boots that had no more body or resilience than chamois despite the boots beginning life with a firm 8-10 ounce cow hide. Beyond that, as the leather loosens and oils build up, yes, dirt and other fines accumulate and even work their way into the corium. And yes, that contributes to the breakdown of the leather structure and cracking.
If you don't care about darkening the leather then I don't suppose it makes much difference how much of an oil based conditioner you apply. But the darkening is a sure tell-tale that the leather is becoming overloaded. If you think about it...when a leather darkens like that what you're seeing is the oil, not the leather.
Thank you for your time, DW. This surely gave me some more insight in how the conditioner works on the leather. I have been less adamant with regards to conditioning over the years as some of mye earlier footwear shows signs of overconditioning, my shoes and boots are now only subject to conditioner sparingly and when in need. I would love to try Bick4, but I still have about half a gallon of Lexol in my closet as the shipping from the U.S. to Norway is often the major cost; I'll have to order some next time.