I've got no idea how they stiffened up. They did, and I wasn't the only one that noticed it.
I live in Texas, so the oil getting thicker isn't an issue for me.
I have the model specific ones, sitting in the garage.
My set of bilsteins have less than 20k on them, and aren't even a year old. They got stiffer, and managed to bounce an extra 2-3 times when hitting average sized bumps. On really big bumps it was just like always, but anything less it was not like it once was. Most of this was coming from the front of the truck also, I'm fairly certain the back ones are still fine.
This is unacceptable to me, I drive on the street unless I pull off onto the dirt to park. The streets around me are mostly all great smooth streets with the occasional one that has potholes and has been resurfaced in random spots when needed. The shocks shouldn't have messed up that quickly, under those circumstances.
When I swapped the Fox 2.0s from the Carli Commuter kit onto my truck, all of those problems were resolved.
I will NOT be purchasing anything from Bilstein in the future.