Brass Adapter, 0.50 Inch Female 42 deg Inverted Flare To 3/8-18 Male NPT, Straight

This adapter is definitely suitable for hydraulic brake lines, power steering, and fuel lines. It's also listed for transmission cooler lines.

NPTF threads like the ones on the male end of this adapter are cut to a tighter tolerance than standard NPT, so they're engineered to form a pressure-tight seal through thread interference alone. That said, a lot of folks in fluid line work still apply thread sealant as a belt-and-suspenders measure, especially in fuel or brake applications where even a slow seep is unacceptable. Nothing in the design prevents you from using tape or paste, and it won't hurt anything.