That's what happens when oil and coolant mix. SLUDGE.
I've been using a very high quality parasynthetic oil since the early 90's, and change oil and filter once a year. That currently means about 7k miles on each of our daily drivers. I have run 10k per vehicle per year in the past.
Last year I changed the valve cover gaskets on our 96 Intrepid 3.5, which has run this oil since my wife bought it in 2000. The valve train was absolutely spotless. I have no doubt that it's cleaner now than when she bought it.
I don't know much about Syntec, other than it's a Group III. Most sludge is caused by coolant leaks into the oil, or OCIs much longer than what the owner admits to.
Bottom line, each person should do what they believe in, because this is a topic filled with so much opinion. If you've got an oil and an OCI that is working for you, stick with it!