Neither one of these. Your car requires SAE 5w30 weight oil.
The 1999 Buick Century Glove Compartment Manual on the inside of the last page shows: Crankcase (Engine Oil with filter change) 3100 Engine 4.5 quarts (4.3L)
The oil capacity for a 1991 Buick Century 3.3 is approximately 5 quarts. The recommended oil type is 10 W 30.
The 1989 Buick Century oil sender is located inside of the oil pan. You will need to remove the oil pan in order to view the oil sender.
Bottom of the oil pan.
what type of engine??
It is important to use the right types of fluid in a car. The engine oil for a 1995 Buick Century is 5w-30 or 10w-30.
The Buick needs 5qts of SAE 5W-30.
On a 1992 Buick Century, the oil filter is located between the radiator and the lower front of the engine. The radiator is situated in slightly in front of the filter.