from what I have heard, it is the POE lubricant not the gas
at high temps polymer drops out from POE, the stuff is too fine for driers to pick up. It drops out of suspension at pressure drops (TXV, cap tubes, etc.)
a more refined POE has been developed that eliminates this problem, but many months before available.
Many guys are replacing the POE with Alkabenzyne and claim it solves the problem (high temp 134a only)
I have gotten the preceding comments from highly reliable sources, but I have no firm undisputed examples. I am not Dan Rather. I do know 2 large resturant equipt suppliers that routinely drain POE and replace with Alkabenzeyne and Shrieve Chem (makers of Zerol A/B) claim it works well in high temp 134a systems.
I've also been told that manufactures are having problems with the POE oils off the lines. They are getting moisture in the systems right on the production lines so it is possible to have plugged cap tubes on new systems!!
ahc&r's comment about plugged cap tubes, jogged my memory about something I had read in the NEWS. I went thru my big stack and found the comment in the Nov 4, 2002 edition on page 16. The guy that wrote in to the NEWS talks of having wide spread problems with cap tubes plugging, and quote-"we began using Sporlan C-052S driers. This almost eliminated the problem". Anyway, I thought I would mention this as this problem has been around for a while now and this contractor had good results with this drier.
Usually on the cap tube systems they use a loose-fill copper-spun drier. These do an inadequate job of keeping the system clean. I would recommend using a Sporlan
C-032-CAP-T to prevent this from happening and to clean up an existing system.