You don't. The oils for freon 12 and 134a are not compatable. There are substitutes for freon 12 out there. I belive on is called sure shot. There very resonably priced.
R134a
1994
R134a
R134a
3.4 pounds of r134a for a day cab r12 is different
A 1998 Buick would have been built with R134a.
R134a
r134a
R134a
R134a freon.........
R134a
R134a