My experience with a couple of old A/C systems that I was converting, is that they both had no obvious oil in the compressor or the drier. They were still working so there must have been some oil circulating with the refrigerant but most had either leaked out with the refrigerant leaks over the years or had gone with the drier changes that were made over the years. The oil to be added depends on what refrigerant you plan to use. I converted to 134a but that was because I ran out of R-12. If you have a source of R-12 at an affordable price, I would go with that and use an oil that is compatable with what might be in your system.

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'97 E 300 D