Wow I didn't think I would open up such a hotly debated subject!
I really appreciate everyone's opinions. As many said, if it pleases/looks good to the truck owner...well that's all that really matters.
I will continue to use the Duplicolor New Ford Grey as that's what I started with and it looks good to me. If I need to do any touch ups, I'll spray some of it into the cap and then brush it on.
The consensus seems to be that engine paint is different than "body" paint and that makes sense.
So again, thanks everyone for your replies.
Brad.