Buick 1998 Century Owner's Manual page 272

What Kind of Engine Oil to Use
Oils recommended for your vehicle can be identified by
looking for the "Starburst" symbol. This symbol
indicates that the oil has been certified by the American
Petroleum Institute (API). Do not use any oil which
does not carry this Starburst symbol.
The engine oil fill cap is located toward the front
of the engine near the yellow-looped engine oil
dipstick handle.
Be sure to fill it enough to put the level somewhere in
the proper operating range. Push the dipstick all the way
back in when you're through.
If you change your own oil,
be sure you use oil that has
the Starburst symbol on the
l
front of the oil container.
If you have your oil changed for you, be sure the oil put
into your engine is American Petroleum Institute
certified for gasoline engines. You should also use the
proper viscosity oil for your vehicle, as shown in the
following chart:
6-12