The only fittings that will need a thread sealer are those that utilize pipe threads/tapered pipe threads/NPT-whatever you want to name them. Typically, a brass to brass or brass to steel pipe thread will seal pretty well without thread sealer due to the soft brass threads that conform to the mating thread. All other threads for liquids and gases such as flare, inverted flare, compression, banjo, etc. will NOT use a thread sealing compound or tape. Those fittings rely on a smooth close fitting TIGHT "seating" of metal to metal and normally one of those metals is designed to "crush" or "form" to make the seal.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy