Lesser-known hydraulic wear often starts with air, not dirt. Tiny air bubbles entering a pump can “micro-diesel” when compressed, creating hot spots that oxidise oil and leave varnish on spool valves. This shows up after cold starts, when thicker oil cavitates more and systems cycle on relief rather than moving load. Moisture adds to the problem by accelerating corrosion and sludge. Choosing a VI-improved AW hydraulic oil with strong air release and demulsibility, and bleeding the system after service, helps. The same principle applies to HVLP grades such as HVLP32, useful when mornings are near freezing.