5w30 any premium brand.
Your "operators manuel" should explain which type of oil to use. I have a 96 Buick, LaSabre, that uses 10/30 wt.
i have a 1996 buick century just came back from mechanic head gasket is gone and there is anti freeze in my oil i guess if you had water in your rad then you would have water in your oil.
If it's later than 99 should be 134A
For the V-6, it takes 5W30.
10W40
According to the 1996 Mercury Grand Marquis Owner Guide : ( 5W-30 is the PREFERRED engine oil weight )
next to the oil pan facing straight down towards the ground
need more information, what kind of Buick and what size engine.
10w30 says it right on the oil cap.
10w30
This engine usually takes 4.5 qts of 5w30 oil.
Right under the power steering pump.