I have been frustrated at the inability to determine if there is oil in an old system. I have converted two old cars from R-12 to 134a, and in both cases I was not able top find any oil in the driers or compressors. Both were still working so there must have been a bit of oil but most had apparently leaked out with the various freon leaks over the years. I have since discovered that I can get a feel for the existence of oil in a system if I open the low pressure valve briefly while the system is running. Since the valve works just like a tire valve, pressing down on the stem opens the valve. The refrigerant and any oil circulating with it will squirt out on your fingers. Once the refrigerant evaporates there should be some oil on your fingers. It is my assumption that having a noticeable amount of oil is all that is needed. I am not an expert, and I am sure an expert will find this subjective oil measurement ridiculous. However, I now have an idea as to how to decide when to add one of those oil charge cans.

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'97 E 300 D