Like Larry said.
Just do not fall for a blend. With names like Freeze 12, FR-12, or whatever. Another poster was recently 'misled' into a blend conversion.
Insist upon r-12(Dichlorodifluoromethane), and make sure they understand.
Also, try to avoid paying for a full evacuation and recharge. R-12 topoffs really are legal, and any shop that says otherwise is misleading you. If it took many years for cooling to degrade to this point, you probably do not have a repairable leak.
1986 300E 5-Speed 240k mi.