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 in the vehicle and immediately start it. The engine rebuild often occurs weeks or even months before it goes in the vehicle so its sitting there in the workshop or car whilst everything else gets finished and gravity takes its toll, draining the oil into the sump. We know of one chap who rebuilt his A Series with machine oil he had, he tucked it away in the corner of the garage and every week he turned it over by hand with a spanner on the crank to make sure it was kept free. He did that for three years whilst he restored the cars bodywork and was surprised to find his newly installed engine had no compression. The machine oil had drained from the rings within hours and the rings had scraped the bores.