OK, tried a little downward pressure, nothing. I tried a wee smear of white grease and it helped a bit but still sticky. Tried a bit of WD-40 and nothing but when I switched to PB Blaster which has a thicker oil component it helped take the squeak out.
It all helped but still not great so I investigated the aluminum recess in which the cap resides and found that it was only finished smoothly on the part you can see. The underside of the lips inside the fuel fill neck was very rough so I put some white grease under there and that was the magic solution. I know it will not last long because the rough aluminum still exists so until I feel comfortable enough to burnish the inside of those lip, probably by removing the neck to do it, the cap will sit in a drawer.
