Graham White is the author of several fascinating and scholarly books on large aircraft piston engines. The two of us were spinning the last fasteners off the magnesium accessory drive cover of a Pratt & Whitney R-4360 radial engine. He ran his eyes over the studs to be sure all were nutless, then slid the plate off them and set it to one side.
Inside we could see the many gears but dimly because the case was well-filled with what looked just like thick, rich beef gravy. This was sludge, which forms when oil is polymerized by heat and oxidation into solid form.