Correct or improve the translation of the following English sentences.原文: The oil used for lubrication must be of correct thickness. 译文：润滑油的厚度要适当。 改译：

Correct or improve the translation of the following English sentences.原文: The oil used for lubrication must be of correct thickness. 译文：润滑油的厚度要适当。 改译：

润滑油的浓度要适当。