They announced the rule for 20+ trainers back in 2013:
Here's a list of some other 20+ Trainers:
As for the W2 Trainers, it will usually be one of the top 3 listed. I used #3 a few months ago for the Steam version and somebody else recently posted it was still working for them on May 12th.
[Edited by element5, 5/27/2015 9:08:25 PM]