I have posted several times on this site the lengthy explanation of the short run sticky valve syndrome. I won't go through that again but I post another item in this thread explaining my reason for discounting the sticking valves in this case.

Synthetic oil is less likely to leave a deposit that is part water soluble leading the "goo" that makes valves sluggish to close and allows hydraulic tappets to pump up and hold exh valves a couple of mm off seat.

Also, wear of the guides (not excessive wear, or even wear anywhere near out of service range) lessens the chance of the sticking valves.

Driving the car aggressively on a regular basis also decreases or prevents the occurrence of this as the valves rotate and do a decent job of cleaning stems at eng speeds over 4000 RPM.