Your new car looks terrific - congratulations! Some comments and suggestions...
Ranger is right: everyone has his own opinion regarding oil, and the studies I've read indicate that it really doesn't matter. Pick any major brand that satisfies you, but change it and the filter every 5,000 miles or as suggested by GM. Unless you drive a tank in the Sahara, there is no advantage to more frequent oil changes.
Depending on your camera, the color of the car might be 8554. Look at the plastic cover over the spare tire, and see if the bottom line has that sequence of digits in it. If it's 8554, Dupli-Color 338 is an excellent match for a non-oxidized surface.
Since jiggling and fooling with the selector gets the car started, your P091 (E091) code is probably fixable with adjustment, as Ranger suggested.
Do not get tempted to "flush" the transmission - a simple drain and fluid replacement (new filter and gasket) will do the job. However, you might want to replace the fluid in the brakes and power steering systems. It's easy, too.
Using the Prestone kit and procedure, flush the cooling system and then refill with simple coolant in the correct proportion.
Pull the simple plate (behind the OPEN lever) on each interior door panel, and lightly lube every reachable moving part in the door lock mechanisms. A little WD40 goes a long way toward restoring the lubricity of the original lubricant.
Drive with an assistant, and using a short length of garden hose find the source of every squeak and rattle - and fix it. "Silentizing" an old car goes a long way toward making it feel new.
Spend some time bent over the hood and carefully inspect all vacuum lines, hoses, belts, mating surfaces (where two pieces of metal come together), etc. Look for cracking, checking, leakage, etc. Inspect the ignition leads (does that car have individual transformers?) for cracking, burning, evidence of leakage, etc. Then, check torque on top and bottom.
While under the car look for broken exhaust system hangers, leaks (tranny and engine pans, PS rack, brakes, exhaust path, etc.), mechanical breakages, and the horrible evidence of a car that's been welded together from two wrecks. Even elderly couples sometimes sell such things, and often don't even know that's what they're selling.
On any old car that you intend to keep, it's a good idea to separate every reachable electrical connection throughout the car, treat with Deoxit (look it up), and reassemble.
Remove, clean, and replace the cables connecting the battery to ground and to the starter (and to the fuse box). Remove and clean the cable that grounds the engine to the chassis - it's usually flat and woven.
Check alignment on all four wheels: tires will give you evidence of a problem. And, are the struts ok? They're cheap and easy, and even electronic struts can be replaced with inexpensive non-electronic parts with little detectable difference in ride. Do NOT buy Bilstein/Koni/$$$ - stick with simple Monroe, Gabriel, etc. at a third the cost.
You ask about performance enhancements, and describe your car as a gas-guzzler. Do not expect that anyone can improve significantly/safely over the GM engineers who developed the car. Just drive it moderately, and you'll get something in the 20s. In the 1,500 miles since getting my car, average has been 22.1.
By the way, in my "new" 1990 DeVille with 22,000 (!!!) miles, I did all of the above, then clayed the exterior and hyperdetailed everything. I also replaced the original Vogue tires (cracked and dangerous), and then installed a radio with a more modern one that includes a USB port (for MP3s) and Bluetooth. It's wonderful to get phone calls over the car's audio system...
Phew!
Now, you and I can drive our lovely DeVilles for another 150,000 miles.
