Fuel Filler Grommet

Anytime you're concerned with a lube reacting with rubber, plastic, or other material type, play it safe and use silicone based lubricant. I like the Sil-glyde silicone lube that comes in a tube, it's labeled as brake parts grease and dielectric grease.

BTW some people here have reported that some aftermarket grommets are hard plastic like. This is the one I bought and it works great:

1967 - 1976 A Body MOPAR FUEL Grommet Gas Tank Seal CORRECT FIT New | eBay