My go-to for tough caulking situations is "Duo-Sil" by "Siroflex". Don't know if it's available in Cal or not. As has already been mentioned, caulking is not for primary weather or water-proofing; it's main purpose is to assist with that and to provide better aesthetics. When you go back 150 years in time you won't find caulking- only putty- and they didn't have water ingression problems. There's something to be learned (or should I say "relearned) with that which is a large part of what's wrong with construction today.
I also have a problem with exterior wood walls, I have a wood cabin home and it was built between the 50-60's. The wall as made with tongue and groove wood logs and now are separating. What can I use to keep it from happening with the rest of the wall and repair the area where it has separated. I am getting ready to insulate the back room and that is how I notice the separation of the logs which are vertical. This is not a siding.