Did my engine have sludge problems?

I saw my 7.5Liter V8 engine without the heads on it because I am getting new headgaskets. While mesmorized of the size of the engine etc, I put my hand on a couple of the pistons, and on the tops of them, there was a pretty descent layer of gritty oil on all of them, I dont know if that was normal or not. Since I have had the engine (since 2006) I do regular oil changes with Mobile 1 synthetic oil, so I dont think I would have been the cause of any sludge problems, and the truck didnt have any sludge like symtoms, but I was still curious though if this was normal or not. I mean, I cant imagine a engine would run too efficiently with that extra film of oil on top of the pistons. Could this maybe even require having to change the pistons out if they have been damaged over the years, or am I over reacting?