I found a grease that makes almost any of the fuel petcocks I have work like new. It is called EZ Turn. It is a fuel resistant grease. I realized I was missing something when I bought some brand new petocks, the old brass type that came on 70s Bonnevilles and Commados. I had always thought these taps were not that good, because I only ever had old ones.
The new ones were beautiful and smooth in operation, they still are after several months use. I realized the main difference was probably the assembly grease which still seems to be hanging in there and allowing nice smooth and leak free operation. I asked a friend who is an aviation mechanic about what this grease was, and he gave me a small sample which would be a lifetime supply except that now I have found other uses for this grease as well.
Now I use it on my paint gun, tolune doesn't seem to wash it away so the needle doesn't stick in the seal. I also used it on some gaskets.
I used it on some 24 year old industrial type petcocks that I have on my Norton and it makes them work like new too.
You can buy that grease from Aircraft Spruce.
Nigel