I have a 1978 Oldsmobile Cutlass with the 260 cubic inch V-8 engine. The owner’s manual specified either 10W-30 or 10W-40. I thought that the added protection of 10W-40 would help the engine, so that is what I intitially used. However, I kept having trouble with detonation (engine ping) from carbon buildup. About every 90 days, I would have to pour a can of Casite motor tune-up in the gas tank and also pour a can directly into the carburetor. On “Car Talk”, a motorist called in with the same problem. Tom and Ray asked about the oil and the motorist was using 10W-40. They said that certain brands of 10W-40 had polymers that extended the viscosity and would cause carbon buildup. I switched to 10W-30 and never had the detonation problem after that.
Now, your Buick is fuel injected and runs much cleaner than my 1978 Oldsmobile. The 10W-40 probably won’t make any difference one way or the other. However, if it were my car, I would go back to 10W-30 at the next oil change.