It should tell you on the air conditioning unit but it's more than likely R12. I have an '89 Caprice and it takes R12.
There is one valve that is used to add refrigrant. But you had better not add Freon. Freon is R12 and your vehicle's A/C system does not have Freon (R12) as a refrigrant. More than likely it uses R134a.
You cAnt sorry
If you are talking about the old R12 freon, you can't legally buy it. The only people who are allowed to buy it are certified air conditioning repair technicians. The government regulates R12 because it is highly toxic to the environment.
They should never be mixed.
Gas used in air conditioners are: R12 freon is dichlorodifluoromethane R134a is Tetrafluoroethane
1993-95 for most.