In my experience, very susceptible. I no longer shoot real BP in the 357 rifle and I'm not worried about the shotgun so my opinion is based on the cap&ball revolvers I shoot. I use both an over ball lube and a lube wad. I have a warm weather wad and a cold weather wad. The warm weather recipe is 50:50 Beeswax:olive oil (or lard). The cold weather Rx is 1 part BW to twop parts oil or lard. The lard gives a harder lube in cold weather so oil is better and it is what I used to use. For over ball lube I use any automotive lube that is available and cheap: lithium based white greases, AMS/OIL Synthetic Grease, AMS/OIL Racing Grease, etc. It is the same consistancy regardless of temp and will not run when scorchingly hot outside. I apply it with a syringe.
For a rifle, my concern would be real hot weather (baking sunlight) getting to the ammo (desert shoots) and melting the lube and having it leach into the powder and causing irregular ignition. Maybe one of our southwestern pards can chime in as to whether that happens or not. In freezing weather your fouling does not remain softened by the lube because the lube itself hardens between stages when the gun cools down and it doesn't all get shot out during the next go around. That's how I see it anyway. The trouble is when you make up your real soft "winter" lube and then use the ammo in the hot summer. That's where it could liquify. Maybe someone will know if SPG is not susceptible to temperature changes.