I love oil questions. I too am new to the website and pending buying my first Cadillac, ever, a 99 STS with 96k miles. However, I literally depend on oil, synthetic to be particular, for my living; I fly helicopters offshore in the Gulf of Mexico and 99% of the helicopters in the world use synthetic fluids throughout the bird. I have also used synthetic oils in my cars, trucks, motorcycles, ATVs, small engine equipment, etc...my whole life. I literally swear by synthetics.
IF you are using a mineral based oil go with the time/distance criteria you establish for your comfort level. I do know that Cadillac specifically labels Mobil 1 on their new 3.6L VVT engines in the CTS and STS. I saw it when I popped the hood last weekend.
I personally would never completely trust a computer to tell me when to change my oil based on the simple fact that YOU had to have reset the DIC data in the ECM since the last change. IF you did that then you are depending on mechanical/electronic time and sensor criteria for a decision in your life.
Remember that your 4.6L Northstar doesn't require an oil change every 3 months or 3,000 miles. The major oil companies do. I personally change my Amsoil every year or 25,000 miles, whichever comes first. BTW Use only a premium filter.
Good Luck and I hope this helped....
JP