My car has the same issue. The ETS light and the check engine light will come on intermitenly and stay on for weeks at a time, then turn off. It is a common problem with the PROM chip in your computer. You can drive the car with these lights on, but you will not have the traction system working. You can take your car to a mechanic and have the computer reset, which is a temporary fix. Or you can replace the PROM chip in your computer.
The transmission is not shifting in and out of gear or your front joint is going bad
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.
theft system
Probably. An auto store would know for sure.