The bellows of the 45mx I recently purchased are misfolded,dry and stiff, so much so that the upper ones do not collapse enough to allow coverage for 4x5 negatives. What can be used on them to "hydrate" them again so they are flexible?
So as a synthetic I doubt that any leather treatment would help.
Fabric inside with a plastic(?) outside. That puts it way outside anything I'd even guess at.
Oh Hell! Guessing never hurt. How 'bout a little hairdryer action to Maybe soften
it to make it easier to refold?
Based on your previous post, I think the problem may be your extended lensboard. Have you put your lens on a flat lensboard? It's been awhile since I had a Beseler, but I recall that the upper bellows didn't collapse as far as the gauge indicated (it came close) but a 150 on a flat board covered easily.