I used PAG 46, but this is in an R134A converted system. R12 would require a mineral oil. The oils are specific to the refrigerant being used as mineral oil is not miscible in r134 and vice versa. Mineral oil for R12 use in cars is standard 525 viscosity, which would match your thick oil discovery. If you have the equipment and more importantly the freon, its not too hard to get your system going again. I found the hardest part to be the snapring holding the electromagnet for the clutch, as the tips on my snapring pliers were either too small or too big. After that the seals were a breeze