What The HD In Engine Oil Means (And Why You Shouldn't Ignore It)

There are few things more important to the health of your vehicle's engine than running it with the correct motor oil. We're not referring to the brand of motor oil you choose but the numbers listed on the label, which convey the viscosity grading of the oil. Your vehicle's engine is designed to deliver peak performance only when the recommended oil is used.

But there are other notations worth paying attention to on the labels of even the best motor oil brands, and the letters "HD" rank among the most important. Those letters can often be found alongside the likes of SAE and 5W-30, among other variations. While you've probably seen the HD tag on a bottle, those labels don't always convey what the letters stand for: a lubricant graded for heavy-duty use.

The HD grading signifies the oil's purpose. Here's why it's so important that you do not ignore those letters when you see them on a bottle of motor oil. 

Understanding the classifications of motor oil, the number and letter combinations found on the label convey its viscosity, or ease of flow at different temperatures. The grade of oil best suited to your car, truck, or SUV is sometimes listed on the engine's oil cap, and not, you can no doubt find it in the owner's manual. 

Should heavy-duty motor oil be recommended, it's likely because your vehicle's engine is diesel-powered, as diesel engines require a different weight of oil than their gasoline counterparts. Likewise, HD helps for engines prone to operating under taxing conditions such as extreme heat, or being regularly used for towing or carrying heavy objects. Heavy-duty motor oils are sometimes also used in the engines of off-road vehicles, as well as those powering some marine vessels.

The difference between a heavy-duty motor oil and one that hasn't earned the HD grading is largely a question of viscosity. Heavy duty oils like Valvoline All Terrain SAE 15W-40 are intended to provide a considerably higher level of viscosity, and thus longer-lasting lubrication than your average oil. Many HD lubricants also boast additives designed to prevent the sort of engine wear and oil degradation that tend to accompany extreme driving conditions, so they're ideal for use in power generators and industrial machinery.