i have an 8 year old mitsu lancer glx with 75k km mileage. i got this varnishing that i can see when the head cover is removed - no sludge though, just varnish.
i change oil 5km interval, sometimes 10km (seldom). i use the turboxp fully synthetic oil from mitsu 15w50.
i want to get rid of the varnishing before things get worse; however, i don't have plans on dismounting the engine or the cylinder head. as of now, the only plan that i have is to remove the oil pan and rocker arms and clean them together with the head cover.
as for the head (camshaft valves, etc) still on the block, i'll just use some carb cleaner (with the valve springs covered to protect the seals) and wipe off the varnish.
once everthing's clean, i'll just put everthing back on with new oil of course prior to reassembly.