On the 2000 Grand Prix, is the oil pan the one in front, or is that the transmission fluid pan?

You have two choices, you can either take it to a shop and have it power flushed, recommended because this gets rid of all the fluid and replaces it with new fluid. The second is to drop the transmission pan, most cars don't have a drain plug for transmission fluid, so you would need to take out all the bolts holding the transmission pan out slowly letting the fluid drain as you bring the pan down. After you have the pan off I would go ahead and change the transmission filter, it is just above the pan inside the transmission. Place the pan back onto the transmission and fill with fluid, about 6-8 quarts.

Depending on how much of the fluid is actually removed the fluid capacities are as follows:
Bottom Pan Removed : 7.4qtComplete Overhaul: 10.0 qtDry: 13.4qt
Since some transmission fluid is generally trapped inside the transmission during service, the actual number will depend on how much of the transmission was serviced.

Most likely cause is the 4th gear clutch shaft splines stripped off,where as fluid will be full,mostly clean and when pan is dropped for inspection you will find moderate amount of fine steel debris on magnet.another scenario is fluid is full,dark in color and smells like burnt oil which would indicate clutch friction disc slipping to the point of burnt.i am making the assumption that other gears happen at proper times and feel good and the check engine light is not on.The most telling trans diagnoses is if pan can be dropped,too much metal or friction debris=rebuild or replace.

You have to drop the transmission pan to drain fluid.

you need to remove the drain plug from the oil pan, make sure the pan plug gasket is not dropped. Then you remove the filter & replace with a new one. After the oil has been left to drain. put the pan plug snuggly back into the oil pan. Refill 4.5 liters of 5w 30w engine oil. Start the engine, check for any leaks from both the new filter & pan plug. Turn off igntion & check the oil level on the dipstick. to make sure it is at the topp off the cross stitched area, if not add more a half liter at a time until it is at the full mark. There is nothing to grease with this car. U should also check transmission fluid, whick needs to be checked while the engine is running in park, there is a hot & cold safe level so check it carefully.

if the fluid is coming from the front of the transmission (near the engine) then it could be a front transmission seal blown .. if its coming from where the axels enter the transmission then you might have a blown spline seal .. you could have damage to the transmission pan .. that happens if a rock or chuck hole hits the transmission pan .. look for damage or a crack in the pan ..

Transmission fluid does not have to deal with nearly the number of problems engine oil has to, with heat, water, acids, grit, etc., so it is not at all critical. In the old days, Ford always had a different fluid because they used different material for seals. But these days the all the fluids sold are great.

As to how much, that depends. There will only be about 4 quarts in the pan and transmission, but the total is more than twice that, and it can very anywhere in between, depending on how much fluid drains down from the torque convertor. It has a little flap that is supposed to stop drainage, but it always leaks down slowly anyhow. So there is never any set amount. You just have to estimate how much came out, put that much in to start, and keep checking and adding, with the engine running.

remove engine mounts and jack up the engine

transmissions do not have drain plugs u must remove the trans pan assembly slowly from one side to the other and have a large container under it to catch all of the fluid then replace the trans pan gasket

Fluid - A/T: Service and Repair
Transmission Fluid and Oil Filter Changing
REMOVAL PROCEDURE
1. Raise and suitably support the vehicle. Refer to Vehicle Lift Points.
2. Place drain pan under pan
3. Remove oil pan bolts from front and sides only.
4. Loosen the rear oil pan bolts approximately four turns.
5. Lightly tap the oil pan with a rubber mallet or pry to drain the fluid from main case.
6. Remove the oil pan bolts from rear oil pan and gasket.
7. Remove front oil pan bolts leaving one corner bolt in loosely.
8. Lightly tap oil pan with rubber mallet or pry to drain fluid.
9. Remove the following remaining components:
^ The front oil pan bolts
^ The oil pan
^ The gasket
10. Remove the oil filter bolts and filter.
11. Clean the following components:
^ The transmission case
^ The front adapter case
^ The oil pan gasket surfaces with solvent
^ Air dry
^ All traces of gasket material must be removed
INSTALLATION PROCEDURE
1. Install the gasket.
2. Install the oil filter.
3. Install the bolts.
^ Tighten the transmission oil filter bolts to 20 Nm (15 ft. lbs.).
NOTICE: Refer to Fastener Notice in Service Precautions.
4. Install the front oil pan.
5. Install the front oil pan gasket
6. Install the front oil pan bolts
^ Tighten the front oil pan bolts to 11 Nm (96 inch lbs.).
7. Install the following components:
^ Install the pan
^ Install the gasket
^ Install the bolts
^ Tighten the oil pan bolts to 11 Nm (96 inch lbs.).
8. Refill the transmission with DEXRON III through oil pan fill plug on the side of the (main case) oil pan. Fill the transmission to lower edge of oil
pan fill. Fill plug opening using and oil suction gun.
^ Tighten the oil pan fill plug to 45 Nm (33 ft. lbs.).
9. Lower the vehicle.
10. Check the transmission fluid level. Refer to Transmission Fluid Checking Procedure.