The main purpose of the lubrication system is to separate moving parts. This helps to:
prevent excessive wear
prevent heat due to friction
protect metals from acids formed during the fuel burning reaction
Without this system, components would start to heat up and melt. If moving parts do melt, they can warp and even weld themselves together (siezed engine).
The image on left shows a ball that will not sink as low in a more viscous oil.
Oil Flow Rate Test
The more viscous an oil, the "thicker" it is. Flow rate is an excellent way to determine how viscous a lubricant is.