what PS said…both seats and valves wear.: .the XK engine does have :“hard” valve seats, but still wear is at both the seats, and the valve edges. The better the “tune’”, the less valve edge wear will be. Stems, tappets, shims, cams all are hardened metal and do not wear much.but will a little. zddp in oil is for metal coating that protects those moving parts that bear, and have oil to them. Use oil that has it. Xk DOHC engines have valve “clatter”. Jaguar tried various means to reduce it due to customer complaints…but it is normal…if it is the normal clatter…not some other noise, all is fine. A tick tick or knock knock is not normal. Valve edge or seat wear will show up as loss of power, sometimes spit or backfire…tested by compression check which if low can be valves/seats or the piston rings, or both. …a wet test will help tell how much is rings.
Valve clearance can be easily checked, but not so easily adjusted,but should be set to be exact…and exactly same on all…so if using say, .006 intake…all intake should be .006 exactly.
Similarly for exhaust, if say, .008. That is what takes some time, and re-do. Jaguar revised the 006/008 to 004/006, likely to try reduced clatter. I choose to use the 006/008 because that is what the engineers established, not the customer service reps later. (IMHO)
Paul S has a super article on valve adj in the Saloon archives.