Good question, Jrod!!!
Heres my take, and this is just my experience, so take it for what its worth.
I know its not part of this, but i'll throw it in the mix - tabs/pills ive used 5-6 years after exp and didnt notice any issues with results and potency (not lab tested though, just by how i felt).
Oils - amps i have zero issue with using way past exp assuming they have been stored correctly. Multi-dose vials im a little more cautious with, i pinned a couple of CC's of test a while back from an old un-opened vial that was stored correctly and it didnt go well for me - mild infection, fever the whole bit, i cleared it up quick with some antibiotics, i dont fuck around with stuff like that. This vial was roughly 4yrs old. Maybe it was a bad vial to begin with, but it made me re-think things a bit. I get it, we can all have a good amount of money wrapped up in gear and you want to get results from it, but for me the risk potential isnt worth it for a vial of test, or whatever drug it is. Just my .02
G