In service since 2013, runs most of the time. Hardly any drop across oil filter. Oil level in separator 40 to 50 %. Oil dp use to be 40 plus, but has gradually decreased and now at 30 to 32 psid. But to maintain that the discharge pressure setpoint was lowered from 160 psig to 150 psig. LOPD alarm set at 28 psid. I did slowly close off the bypass to see if there was any overflow and no change in pump pressure. Last oil sample had a small amount of ferrous wear like the previous samples (8 to 11 ppm). Probably going to suggest a compressor inspection (or rebuild) which HOWDEN recommends after 4 years (not my specialty)!