Damn!Pulled the head.
Lots of deposits. Limestone formed clusters as high as 1/16" to 1/8". I blame Mobil 93 gas or Red Line High Performance oil. Was using those two almost exclusively for past 3-4 years. Valve stem seals were old, combustion chamber saw a lot of oil, so that limestone could be additives found in Red Line in large quantities.
I think I'll just go ahead and replace all exhaust valves, just to be safe... And switch to Shell and the low-ash Red Line Euro-Series.
Otherwise, the engine looks super-good for its 220k miles. Donor engine with the same mileage was worse inside when I took it apart last winter. Mine saw synthetic oil only every 5-7k miles since 80k and lots of coolant changes - coolant passages are clean, cylinders have hone, pistons have no play (besides the cylinder upper part, where the slots between cylinders are cut). Still have to clean the head thoroughly, check for warping and measure the cylinders.