Sorry to weigh in so late. Missed this one as it came by.
We have experienced this problem many times. Almost always after a plant has changed to a different type of oil.
My understanding is that in the last few years many Oil Suppliers have increased the solvent content of their oils. This affects the O rings causing them to fail.
The Rubber parts in a seal may change from time to time. An old stock seal may use a different formulation than current production.
Another problem that can occur is if you let the Seal parts contact solvents. (This happened to me) Automotive Brake Cleaners which many people use can cause some O rings to swell on contact.
I have also noticed a lot of Manway gaskets on Screws being very badly swollen lately on NH3.
According to information we have, Manway gaskets on ***** systems must be of VIRGIN Neoprene to avoid deterioration. I recently had a problem and cut my own out of 60 Durometer Neoprene.