Seems that you might be fine since you have the updated seals.
Keep in mind valve seals are an internal engine part...the rubber is made to withstand high temps. Your engine and oil cooling system should keep temps in a range where they are meant to run perfectly.
Going with a lower thermostat will affect a lot of other things and will not fix the valve seal problem.
Keep in mind valve seals is nothing new with engines. Focus on things that are new will point to the problem...for example
-Direct Injection
-Extended Oil Intervals
-People wanting to cold start the car and drive
Out of these 3 things direct injection may have the most affect on seals. Prior to direct injection, intake valves were always cleaned by the fuel injectors. Removing this allows build up from the valves from the combustion chamber. There's also build up from the crankcase ventilation system constantly coating the area with oil film.
Walnut blasting is a common procedure with direct injection engines. The reason it's not marketed for our v8s is because of the labor cost involved. (most cases the engine needs to be removed)
So just because we have intakes on the bottom side of the engine doesn't mean we're immune to the disadvantages of direct injection.
https://www.newtis.info/tisv2/a/en/f...old/1VnYvs8OBw
Installing an oil catch-can will help stop the build up of oil and film in that area.
Not sure about this but, water/methanol injection may help with the cleaning.
Shortening the oil interval should help renew the additives lost over time. Some additives are made to help swell up the rubber valve seals. Though you have to constantly use these additives...it's not a use once-fix-the-problem-forever.
Personally from a cold start I let the car sit at least a minute before considering driving off. Synthetic oils are much better than conventional oil so you may not need to do this.
One of the best things to do is spend a lot of money and have walnut blasting done
