What Oil to use on O-rings?

When replacing A/C system O-rings, it's always recommended to lubricate all O-rings with clean 525 viscosity mineral oil.  Oil on the O-ring surface helps protect the O-ring during the assembly of fittings.  The use of PAG and Ester oils are not recommended for this purpose.  Many PAG and Ester oils attract moisture, which is said to promote corrosion at the fitting. 

Sealing washers which are common on General Motors vehicles were designed to be installed dry. Sealing washers do not require any lubricant during assembly.  Lubricant will not affect the seal, but the chance of dirt sticking to sealing surface during assembly is why lubrication is discouraged.   

Nylog works very well as automotive air conditioning assembly lubricant.  One bottle of Nylog "Red" or "Blue" can be used to lubricate many hundreds of O-rings.  In addition to its' ease of use, Nylog is manufactured from refrigerant grade lubricant, so there is no chance that the product will cause any contamination.

As a matter of personal choice, I prefer the "Blue" variety of Nylog  but found both work very well on automotive air conditioning O-rings.  Fittings that were lubricated with Nylog were found to still have traces of the product after many months of use.  This would suggest to us that the sealing qualities of the lubricant remain intact, even in the high temperature and pressure associated with the automotive air conditioning system.