What do the different ratings mean?

Piston rings to scrape the oil, block the explosion from getting down past the piston.

What does oil encounter during its lifetime (cold start, hot operating -- changes in viscosity).

If you use different metals when building an engine, you'd better be sure that they expand at roughly the same rate. If you make the piston from metal which expands faster (as the temperature increases) than the metal which the cylinder is made of, you'll have problems.

If you've got a rotating shaft, you need to support the two ends of it somehow. The two ends get supported in bearings (since they bear the load) in the engine case. A bearing is just a specially chosen material which reduces the friction between the parts - such as brass. You still need oil though.