Yes, there are shelf life issues with POE lubricants. Two to four years is the normal shelf life of POE lubricants, but the product may last longer if it was well sealed and kept reasonably cool. I don't remember if Frick has set any limits of their own or not. CPI / Lubrizol may have better information on the shelf life since they made the Solest 68.
You will need to test the moisture content and TAN values to look for degradation. The moisture should be less than 100 ppm, preferably less than 50 ppm. The TAN shoud be less than 0.1, and preferably less than 0.05. If the POE fails these tests, you are taking some risk by using the product.
There should be antioxidant in the product as well, and if it is still working the color should be less than 2.0 on the ASTM D-1500 scale. If the color is darker than 2.0, there is likely to have been oxidation.
