probably a vacuum leak check all the rubber hoses about 1/4 inch in diameter and hard plastic hoses about 1/8 inch the hoses that go to the vacuum switch that works your heater/ac vents is also prone to leak turn selctor to max then norm then defrost and listen for a spewing sound you should hear it for a second or two each time you move the selector knob if the spewing continues it is leaking
The transmission is not shifting in and out of gear or your front joint is going bad
It could be the tires out of balance, the wheels need an alignment or the steering mechanism needs service. Can also be a bent wheel, but I would suspect it is an out of balance tire.
I would say that you are losing coolant somewhere hoses,water pump,radiator.
I'm not sure, but it may be time to have the transmission serviced.
I would go with 10w40.
Try changeing the fuel filter,I had the same problem and it helped.
A 1998 Buick would use R134a refrigerant.
A lack of oil in the engine./ the sensor in the oil system./ broken oil pump
mission impossible
it sounds like a steady bearing on the drive train or pssibly your tourque converter is going bad
A GM dealer's service center would know.
Probably. An auto store would know for sure.