Welcome to the site.
I hope you enjoy your new DTS as much as I have enjoyed mine.
There are a lot of people here with knowledge...I am not one of them.
I just have a lot of opinions.
There was a major re-design of the headgaskets in 2000.
I have heard very little about newer cars having the problem.
That don't mean you can ignore the cooling system. Based on past problems with the older models, I will be servicing mine a little more often than the book says to...just to be on the safe side.
As for the best oil...Everyone has an opinion on THAT...
Starting in 2007, I think Cadillac switched to Synthetic oil in the N*.
A lot of the people on this forum, are running Synthetic oil.
The DTS has a built in OIL LIFE MONITOR, that will tell you when to change the oil. It monitors your driving habits...speed...rpm..etc ..etc and calculates when the oil needs changing.
That being said...I am still running regular oil in mine, changing about every 5000 miles. That is usually around 50 percent on the oil life monitor.
Texas Jim