Replace the hydraulic oil once at least annually; and only the high quality oil should be chosen.

液压系统中的液压油应至少每年更换一次，液压系统只能选用高质量的液压油。

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The soot-blowers should be disassembled and inspected every two years. The carriage must be changed HJ-30 engin oil once a year.

吹灰器一般每两年解体检查一次，跑车每年换HJ—30机械油一次，注油量应与正常油位线平齐。

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Once its impurities had settled, the oil could be graded.

一旦杂质沉淀下去，油就可以定级了。

《柯林斯英汉双解大词典》

As credit markets recover, he says, traders will bid up oil once again.

FORBES: Crude Cassandra

George Osborne believes that cheap gas could boost the economy just as North Sea oil once did.

BBC: Why gas prices are so politically hot

The crises in various the smaller oil producing nations occurred once oil production and revenues dropped and subsidies declined.

FORBES: Saudi Regime Isn't The Culprit NYT's Friedman Claims, Says Prince Alwaleed