So, some years back the original cork/rubber gasket on my gas cap kind of fell apart. When ordering a bunch of small parts from one of our suppliers (not nameless, I just don't remember!) I bought a rubber replacement, it was a distinctive blue in color. That was always too snug and gave me fits when replacing and locking the gas cap. So, a year or so ago I bought a better replacement from another one of our suppliers (still trying to find a receipt!). That fit well.
Yesterday, I filled the tank and lo and behold that gasket was soft, mushy, had swelled, and when I tried to push it into place on the cap, it started to come apart. Awful smell of fuel on the drive home from the gas station as I'm sure some fuel was coming out the back as the gasket was no longer working.
I don't have time to wait for a replacement so I ordered some specific fuel resistant gasket material from Amazon (cheaper than just the shipping from one of our vendors here) and will cut my own.
I'll amend the post here with photos when that arrives.