The oils are definitely incompatible.  When my system was converted, the mechanic who did it washed the system with a solvent that was supposed to remove the mineral oil before he replaced it with the ester oil required by the R134a.  Does the compressor come with the oil already in it, or does the oil have to be added?  If it comes dry, requiring oil to be added, then you've no problem.  The label is simply from the old days.  But what do I know?  The compressor could come already oiled.  mcneely

---- J Stewart <fonman4277@COMCAST.NET> wrote:
> Then, if that is true, the compressor Bus Depot just sold me as compatible with either R12 or R134A isn't-the label on it says "mineral oil" I'm still confused here and don't want to ruin an expensive part! Jeff Stewart ----- Original Message -----
> > On 7/8/2012 9:05 PM, Dave Mcneely wrote:
> > > Hmmm..... . The AC Pro stuff I saw today at AutoZone is R134a. The
> > > fittings for R134a and R12 are different of course. If your vehicle
> > > has not been converted, it is R12.
> > Good catch. R134a systems are required by law to have different
> > fittings, which would point to it being an unconverted R12 system.
> > R134a
> > requires a different lubricant (PAG or ester), because it won't
> > effectively carry the R12 lubricant (mineral oil) through the system.
> > The result is a ruined compressor.

--
David McNeely