R134a runs at higher pressures than R12.

Polyalkyl glycol lubricants can have a problem with chloride compounds such as CFC's and HCFC's. Any residual chloride left in a system has the potential of reacting with the PAG. R12, R22 are examples of these. There are DOUBLE END CAPPED PAG lubricants (Daphne is one brand, Japanese, I think) that many say will work with R12, but I would not use the plain old PAG. Double end capped referrs to the fact that the reactive ends of the Pglycol molecule are chemically "capped" to prevent halide (perhaps many anion) reactions.

Some would respectfully disagree with using ANY PAG with R12. I'm one.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)