I am missing the "on the inside" in several posts above. Oil is all on the outside, except for the oil passages. Water coolant is on the inside. I should amend that slightly for the slant since the lifter galley is a bit sunken, but still accessible if a "drool tube" head (gee, the O.P. didn't tell us what motor he has, why do they always do that and expect answers?).
This is not atypical. I have seen photos of some newer Japanese engines where the valve cover was removed and the oil gunk formed a solid mass molded to the shape of the valve covers. Relative to that, 1/16" thick is not much.
My 273 was a bit gunky on top, but much less than yours. I removed the oil pan (jacked engine up 2"), brushedd the top (rockers, lifter galley) with gasoline until it was shiny and reassembled. I didn't remove the heads. The only engine work I did was change the timing cover, timing set, and rear main seal. Engine runs great now. Do not use a steel bristle brush, plastic or copper at most (and only an idiot would use sandpaper).
I must do better maintenance than most because whenever I remove the valve cover on any of my cars, classic or modern, it looks new under the valve cover. There are some amazingly negligent owners out there.