Is this a commercial refrigeration or cimco rack? are the heads water cooled? often times these are mycom recips with water cooled heads. when the water passages are not cleaned as part of a pm program, the compressors run hot. The oil sperators they use have very little tolerance for high discharge superheat, and the element comes apart internally and stops doing much work. . I assume this is welded into the system, and there is no way to take it apart.
Any way, you may actually have an issue with the seperator, and the oil is getting carried through the evap condenser and then back to the evap. trap could be just fine and clear, and appear to be working ok.
Then again, I could be waay off base.