> I've read that the old oil can't be fully
> removed, and when you add the new oil, they mix and make a
> difficult-to-remove coating, which helps to seal the pores in hoses.

My version was "the old oil has sealed all the pores over the
years...."  Hard facts are really hard to come by with AC.

I converted my '87 from non-functioning R12 to R134a.  They
tested for leaks, then evacuated and converted.  It worked fine--
plenty cool--for 2-3 years until one winter when I could not
run it monthly.  The compressor seals dried and it leaked.
New seals, and now it has run cold for a couple of summers.

One of my smug pleasures is running the AC while grinding along
some Syncro road in granny low while the ATVers eat dust.  ;-)

Running monthly year-round is one key to staying cool.

Richard