90 740Tic (just rolled over 280000 :D )

My AC is pretty wimpy. It seems like it can't cool the heat sink (term?) that cools the incoming air fast enough. It will get very cold, but once I've had the fan on high for a while (this is at highway speeds), the incoming air warms up. If I turn it to low speed for a minute, when I turn it back to high, the air is nice and icy....for a few minutes. Then it's basically just cool air.

Seems to me like it's a bit low on refrigerant >.<
at 280,000 miles, it makes sense.

I'm the car's fourth owner, it may have been switched to R134a - is there a way to tell?
Is it completely necessary to convert over to R134a if I want to add more refrigerant? It seems to work about 60%. If I add Ester Oil and R134a, do I really need to remove the old oil and refrigerant? I repeatedly saw "PEG oil" or something that will interact with mineral oil and wreak havoc..... but is there one or the other necessarily present already?

Can someone clarify for me? I've recharged two other cars' AC and it was ridiculously easy.

Thanks,
Will
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1990 740 Turbo, on its way to stock specs, maybe beyond