Engine oil seems so simple. It’s slippery. It keeps metal parts from fusing together. It’s messy, especially when it has been squeezed, slung, and sprayed around the inside of a hot engine for thousands of miles. And after it has been inside an engine for a few thousand miles, it seems dirty, worn out, and ready for disposal. But engine oil never wears out. It gets contaminated from combustion by products and it loses its additives to heat and pressure, which is why it must be changed from time to time. Otherwise, oil lasts virtually forever.
Click on the link to read the complete article 
New Motor Oils Are Bad for Older Engines with Flat Tappet Camshafts –The Zinc Factor