Heading 3819 applies only when the fluid contains less than 70% by weight of petroleum or bituminous mineral oils. Fluids at or above that threshold are classified in Chapter 27 as lubricating preparations or other petroleum-based products. Importers should obtain compositional data from the manufacturer to confirm which side of the threshold their product falls on.

Aviation hydraulic fluids based on synthetic esters or polyalphaolefins with low petroleum content typically fall under 3819, provided the petroleum content is below 70% by weight. Phosphate-ester-based aviation fluids also generally qualify. However, if the formulation is predominantly mineral oil, it would shift to Chapter 27. Always verify the base-fluid chemistry and petroleum content before classifying.