Many hydraulic failures start with trapped air, not simple part wear. Tiny bubbles compress and heat, causing microdieseling (air ignition) and varnish. This risk grows during cold starts when fluid is thick and inlet restricted. A fine suction strainer often makes it worse by starving the pump. Use coarse inlet screens, and keep pressure filtration on the return line. Watch for slow actuator response and milky sight glass, both suggest aeration. Short warm-up with low load helps release air and prevent cavitation. Correct viscosity grade improves air release time and pump case drain flow. For reliable operation in Nordic workshops, choose fluids known for stable behavior. See practical details and test notes in the listing for HVLP68.