it's a mineral based oil
Mineral HD oils usually carry a higher TBN number than a full synthetic
The higher an oil’s total base number (TBN), the better its ability to neutralize contaminants such as combustion by-products and acidic materials. TBN is a measure of (alkaline) additives in the oil. Higher TBN oils are able to neutralize a greater amount of acidic materials, which results in improved protection against corrosive reactions and longer oil life.
TBN levels are targeted for the intended application. For example, gasoline motor oils typically display lower TBN numbers, while diesel oils must manage the high contaminant-loading from soot and sulfur and typically have a higher TBN.
TBN levels decrease as the oil remains in service. When the level reaches a point where it can no longer protect against corrosion, the oil must be changed.
Oils that are formulated specifically for extended drain intervals typically display elevated TBN to ensure proper corrosion protection for the duration of the extended interval.