I actually thought about you yesterday when checking my fuel pressure I was looking at the same type of clip/sleeve on the fuel line
My trans cooler line was leaking a fair bit at the same place when I bought the vehicle but stopped after I serviced the transmission with the Valvoline extra life fluid that I think has a bit of conditioner in it. I wanted to try to replace the o-ring but couldn't get at the sleeve to release it with the tools I had so I ordered a different release tool and even had the trans cooler line in my cart for purchase but ended up not using any of it. Kudos for posting the o-ring size!
I think there is an o-ring size chart on here somewhere would be nice to have that on there and have it a sticky or some reference to it.