I still use white gas stoves for car camping, and have a bunch of old Sigg, MSR, and other fuel bottles. But the o-rings on almost all of them are dried by now, I purchased them in the late 1970s and 1980s (no complaints, they lasted a really long time).
I purchased some no-brand o-rings on the web a few years ago and they are all now cracked. They were sold as correct for fuel bottles. They were the correct size but apparently not the correct material.
But what is the correct material for white gas? And what might have been the material of the ones that cracked quickly?
The one on the right was barely used, maybe a few times to pour fuel out, it cracked anyway.
One caveat: the original o-rings were flat, the ones I purchased were circular in cross-section. Could that be part of the problem? Seems unlikely as even the unused one cracked.
Some pictures that will stir the hearts of old-timers, particularly the Canady Pouring Cap:
