Unless the surfaces that the oring will be sealing against are a 16 microfinish or better, orings need to be greased. A very thin, light coat that is barely perceptable is sufficient. Either a hydrocarbon (Apeizon M) or silicone (DC 976) grease will work. Hydrocarbon greases easy to remove. Silicone greases and not as easy to remove and can be a source of contamination in semiconductor processes. However, it is very inexpensive.
Related Topics: Vacuum Grease