Proper PAG oil for a/c system?

It seems there are a few viscosities of PAG oil made for a/c systems. As far as I can tell, Ford uses the low viscocity oil for their system. Can anyone confirm this? I picked up a used (crosses fingers) compressor for the van and will be installing it this weekend.

TIA

It is a lower number ... I don't remember exactly which. My Winny is not here at the moment, but I believe the foil tag underhood has that specification written on it.