reduce friction, cooling & surface abrasion (wear) between moving parts.

modern jet engines

dermatitis

is the lowest temperature which fluid can be ignitable mixture with air

is the measure of fluid resistance to flow.

low enough for easier starting at low temperatures and high enough to keep lubrication at high temperatures

globules or misty.

breakdown of seal within the lubricant system

allow the oil to hold sludge or debris in suspension to prevent its buildup within the engine

take oil sample clean or replace oil filter and flush system with clean lubricating oil

high heat energy per gallon and lubricate fuel system components.