"We change many of our lubricants according to a schedule, ranging from six months to two years, depending on the lubricant and application. Because we use a scheduled oil change criteria (instead of a condition-based oil change), we are uncertain as to whether the time interval we have chosen is optimum. How do we know if we should extend or shorten the scheduled interval?"

In many cases the original equipment builder can suggest helpful guidelines in this area. There may be useful technical support information available from your lubricant supplier as well. 

However, because a lubricant's service life has a lot to do with the stressing environment it is exposed to, the best way to tweak your oil-change interval is to make actual assessments of its condition and remaining useful life.  Unlike routine oil analysis, the types of tests you might want to select could be more similar to what you see on your lubricant's spec sheet.

Begin by talking to your lubricant supplier about which performance properties would need to be evaluated. These could include oxidation stability, rust/corrosion protection, air release and foam stability, demulsibility, antiwear protection, VI (Viscosity Index), silt particle concentration and other

essential performance properties. Expect the cost of these tests to run several hundred dollars.  However, you only have to run tests on samples from a few representative machines.

Obtain the sample just prior to the currently scheduled oil change. Send this sample along with a sample of the new lubricant (for baselining purposes) to a lab that can perform ASTM performance tests. You may need to obtain several hundred milliliters of fluid of each sample.

Once the tests are complete and an assessment made on remaining useful life, a decision can then be made on whether the oil-change interval can be shortened or lengthened. A reasonable safety margin needs to be included in the decision. Once changes in the drain interval are made, monitor the oil carefully.