looks like about $200 for a new compressor, so not crazy money..... one question for you as you are mid process.... is the expansion valve inside the car?You are literally into the same job I'm halfway through right now. You've found some better Sanden literature than I managed to find. Thanks!
What I've managed to piece together so far...
1. You need to flush all the old oil form all the lines. I'm halfway done with that now... got slowed down by running out of flush solvent. The walkthrough above doesn't seem to indicate a thorough flushing of all old oil. But everything else I've read says you need to get most of it out.
2. You need to replace your expansion valve with an R134a unit... your current valve is likely R12 specific. And you can't run flush through the expansion valve. The walkthrough above doesn't mention anything about the expansion valve either. But a new Denso expansion valve is only around $25.
3. You'll need a new drier bottle. Covered above.
4. You'll need to drain as much oil as possible from the compressor. Covered above. Sanden 508's take standard PAG 100 as their replacement oil. Sanden has their own name for it above, but I found another document that listed the PAG 100 as suitable for the 508's.
Mine worked like a champ before it was discharged to swap out the front radiator support. So, like you, I'm trying the conversion. If that fails then I'll source a new R134a compressor.