What type of motor oil does a 2000 Grand Prix use?

You do not mention your engine size.

However for a 3100 engine the GM manual specifies a SAE 5W-30 for best fuel economy and cold weather starting.

For a 3800 engine or a supercharged engine the manual specifies

SAE 10W30 for temps above 0F (-18C) and if it is going to be very cold

SAE 5W30 for temps below 60F (16C) and below only

Hope this helps.

"G"