Medium with worse lubricating property, such as light fuel oil, heavy fuel oil, coal tar, asphalt, viscose, emulsified mixture, etc.

润滑性较差的介质，如轻质燃油、重质燃油、煤焦油、沥青、粘胶液、乳化液等；

youdao

Already, the Obama Administration has made historic progress through investments in clean energy, new fuel economy standards for cars and light-duty trucks that will save approximately 1.8 billion barrels of oil, and steps toward the first-ever fuel economy standards for medium- and heavy-duty trucks.

WHITEHOUSE: President Obama to Visit Indianapolis

Unlike the light-duty vehicle market, most medium- and heavy-duty truck purchasers operate on slim profit margins where fuel costs are a major factor in their economic survival.

FORBES: Magazine Article