There have been a lot of questions about the propane/butane substitutes
for R-12 (ES-12, Duaracool, etc.) so here is some additional
information.  A close friend of mine owns and runs an auto repair shop.
He has done quite a bit of work for me on both of my Westy's (85 and
90).  When the AC quit cooling in my '90 two months ago, I asked him if
he would work on it, he said "No, I don't work in AC anymore".  Last
weekend he explained why.  It seems that he had wrecked his $6000 super
duper AC machine twice when people had failed to tell him that they had
done a shadetree job on the AC with the propane/butane mix.  It
apparently can develop very, very high pressures, and that had cost him
two repair jobs on his equipment.  After the second repair, he sold the
machine to someone else.  His other alternative was a $3000 machine to
tell him what kind of coolant was already in the system.
So be warned, if you use the propane/butane mix, and then go to a pro
shop for work WARN THEM!!
BTW, when I told him I could prove with receipts from the PO that the
last fill was R-12 (a year ago) he said OK, bring it in.  He still has
all the basic tools for working on AC, just not the fancy equipment.
He also pointed out that AC is one of the most customer antagonizing
jobs because so oftem there are several components that will need
replacing but it is very difficult to determine which ones except one
replacement at a time.
Can be very time consuming (translate high labor bill) and impossible to
give accurate estimates.  Probably more of a problem here in the
Nortwest where we so rarely use our AC that it deteriorates and then
when we decide to have it serviced we get hit with one big bill.