A common problem on many General Motors cars is the Torque Converter Clutch fails to release and causes the car to stall when it comes to a stop or when started and put in gear. Most of the time it is a stuck Torque Converter Clutch (TCC) solenoid, but this is not the only cause of this problem. Generally, if you unplug the TCC solenoid at the transmission and the symptoms go away, you have found the problem. But sometimes this can be misleading because you don't know for sure if it's a bad solenoid, dirt in the valve body or a bad signal from the ECM. I would suspect you will find the solenoid is bad or has been unplugged.
Could be a vacuum leak. Check the PCV pipe and replace any air hoses.
I would go with 10w40.
A 1998 Buick would use R134a refrigerant.
mission impossible
A GM dealer's service center would know.
theft system
Probably. An auto store would know for sure.
Bad head gasket Cracked head Cracked block
The transmission is not shifting in and out of gear or your front joint is going bad
there could be flood damage, I would not recommend it.
Go to a GM dealer's parts department or service department. They would know.
Well the fast answer is "whichever one fits", but any hardware store would have them.