This Macmillan dictionary suggests that the when/whenever/where/wherever possible are synonyms:
when or where you have an opportunity to do something. I relax with a good book whenever possible.
However, I've seen this answer (it seems from a native speaker) on this forum.wordreference.com post:
It seems obvious to me that "when" refers to time and "where" to place.
Call me when possible. (= call when you have time)
Walk where possible. (= walk wherever you can or are allowed to)
In the above exercise the obvious answer should be "when" because it refers to time, unless I'm missing something.
It suggests that they are different. I'm confused. I've also seen this example from the Potter book:
Always brought people in alive where possible.
In this case, can we put: Always brought people in alive when/whenever/wherever possible.? Would they mean the same thing?