Sending unit leak - gasket question

Hey, my tank is new, and the sending unit is new, and it's leaking at the sending unit. I used the neoprene gasket and lock ring provided. I read in another thread that the original lock rings are thicker than the new ones, and to use them. I still have the old locking ring, so I'll try changing it over.

My question is about the gasket. Should I be using the old rubber gasket? Or the new neoprene one? I also have a sheet of gasket which I can cut out to make my own, although it's pretty thin. I could use a gasoline resistant RTV to bond 2 together to thicken it up, but that sounds like a bit of a hack job.

Thoughts?