I'm having an oil situation here at a facility and wondered if anyone could throw some light on it.

In one of our facilities, we've seen viscosities between 85-91 for several years from our compressor oil. The oil is Clavus 68 from Shell. When I test it new, I get a viscosity in the 65 range. However, all six of our compressors regardless of duty have oil viscosities in the 90 range.

What could be causing this? I am not seeing this increase in viscosity in the other facilities although none of them:

1) Run anywhere near as hard
2) Run in a vacuum
3) Are using the Shell oil

We use an autopurger - we routinely test the oil and have found no contamination at all. Of course, we have also seen no sign of anti-wear additives in the oil either.

I understand that the recommendation is to replace oil when it varies 5% from the initial viscosity. That's a very expensive proposition when you are talking about 6-700 gallons of oil.