Traditionally, oil was the go to for engine rebuilding. Camshafts, bearing and cranks were all doused in it to aid assembly and prevent metal to metal contact prior to oil flow and pressure coming on tap a few seconds after start up. Thing is, most classic owners don't rebuild the engine, put it..
The famous American sealing compound which has been around for years and many folks swear by. The lovely, traditional, 4 US ounce (118ml) metal can is plenty for do-it-yourself automotive enthusiasts as it is very fluid. Use on gaskets but manufacturer also advises it can be used on hose connections..