Hitachi ZW 550-6 Operator's Manual page 326

IMPORTANT:
 When connecting hydraulic hoses and pipes,
take special care to keep seal surfaces free from
dirt and to avoid damaging them. Keep these
precautions in mind:
 Wash hoses, pipes, and the tank interior with a
washing liquid and thoroughly wipe it out before
reconnecting them.
 Only use O-rings that are free of damage or
defects. Be careful not to damage them during
reassembly.
 Do not allow high pressure hoses to twist when
connecting them. The life of twisted hoses will be
shortened considerably.
 Carefully tighten low pressure hose clamps.
Do not overtighten them.
 When adding hydraulic oil, always use compatible
oils; do not mix brands of oil. The machine is filled
with Super EX 46HN when it is shipped from the
factory. When selecting to use another brand of
oil listed in the table "Kind of Oils" - "Hydraulic
Oil", be sure to completely replace the oil in the
system.
 Do not use hydraulic oils other than those listed
in the table "Kind of Oils" - "Hydraulic Oil".
 Never run the engine without oil in the hydraulic
oil tank.
MAINTENANCE
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