We have oil samples from these units checked for viscosity, water, TAN, and wear metals.
Can anyone give us a guide to the limits we should work to.
TAN of new oil varies from 0.01 to 0.29 depending on oil supplier.
Thank you.

You need to get more information. What refrigerant and oil type and compressor type are very important in order to set good limits.

I have never seen a refrigeration oil with an initial TAN of 0.29. They are usually low TAN (less than 0.05) if additive free, or very high TAN (greater than 1.5) if it is a rebadged hydraulic oil.

The compressor OEM may be able to give you guidelines on the acceptable oil analysis results, as long as you are using an approved oil.

Thank you for your comments.
Regards, Richard

I do some analysis on some Castrol icematic oil, it normally has a TAN value of <0.02.