Engines are lubricated almost immediately. The oil pump is typically camshaft driven, so it's only a matter of seconds before everything is lubricated. In addition, modern oils tend to "stick" to parts better and don't drain off as easily. It takes quite some time before they're completely dry.

As far as the material, at one time engines had components of various materials. Blocks of cast iron, heads or intake of aluminum. Now as you state, they are mostly aluminum. While it may expand more than steel, the majority of the components are going to expand at the same rate, unlike in the past.