That's what I thought too, until I set up some "heat stress" tests, albeit on color films.
This post is carried on from my earlier post, #67. In order to see how sensitive the pro color neg film of the time, VPS III, was to higher temperatures, we used a "hot box" set to 140 deg F (60 deg C). The way we arrived at 140 deg was to roughly mimic the highest automobile interior/trunk temperatures we could imagine, plus some.
Our film included both 35mm and 70mm VPS III, plus a couple other color neg films for reference. Two of the reference films were amateur grade, because like you said, the amateur film ought to hold up better than the "delicate" pro films. The method was to run sensitometric wedges on each of the films at the beginning, and then at some arbitrary intervals afterwards.
Results? I don't remember exact details, but we started out with smaller intervals, like 4 hours, then kept increasing in progressively larger steps. Nothing showed any change until somewhere over 250 hours or so, whereupon the amateur films started to go bad. The "delicate" pro films went quite a lot longer, perhaps 350 or 400 hours before they exhibited the first sign of change. Once change started, on any film, it got continuously worse over time. I should be clear that I don't remember exact times, but these are roughly in the ballpark.
We didn't test further, because this was pretty conclusive to us that none of our photographers were going to accidentally damage their film by keeping it in their car. First, none of the car interiors were going to get within 20 degrees of that hot, and second, even if they did, it's only during sunlight hours, a nominal half day. So our nominal 400 hour break point for the pro color neg film, roughly 2 weeks, would more closely represent double that, a month, during real days.
I would not want to extrapolate these results to other films. But if you think that amateur films are more robust than pro color neg films, then this is a case where you would be wrong!
Again, sorry I can't say anything about B&W, the real topic.