"I have learned that high temperature encourages additive depletion and base stock oxidation in bearing oils. We are a paper mill and have been told by the paper machine supplier that our main lube oil reservoir should be maintained at 180 degrees F. Why are these high temperatures recommended, considering the harm it can cause?"
You are correct in stating that high operating temperatures encourage additive and base oil failure. The widely used rule of thumb is that for every 18 degrees F increase in temperature, the life of the oil is cut in half. Still, it is common for high temperatures to be recommended in paper mill applications.
The reasons include better water-shedding ability, lower foaming tendency, better air release, improved particle settling rate, better drain back flows and more rapid vaporization of water. However, most of the benefits from increased temperatures can be sufficiently achieved at, say, 140 degrees F.
At this lower temperature, viscosity will be higher. Therefore, you might want to revisit the viscosity grade selection. Generally, lower viscosity oils are more resistant to oxidation and thermal failure. It is good advice to discuss a proposed temperature and/or viscosity change with your machine and oil suppliers.
Because temperature plays such a vital role in machine condition monitoring, heat guns are found in most PdM tool boxes these days. Just like we need to take our temperature to know if we are running a fever, most problems with lubrication, friction and wear will have a temperature profile or signature. So in that sense, temperature change is good.
Like many things in life, when it comes to lubricant temperature, there's a need for control and moderation. In other words, you can expect problems if you have too little or too much. Find the temperature sweet spot, and the performance and service life of your lubricant can be extended. Of course, it's all so easy to say, but in practice can be oh so difficult to do.