Blood maintain the whole body,keep running all moving parts and show the status of full body.Just like this lubricant,keep balance of speed,temperature & other important perimeters of the engine.Though basic function is to reduce heat & friction in engine but lubricant maintain the whole engine’s control. That’s why lubricant called the blood of the engine. But important thing is that, when a body is affected by any diereses then it’s symptoms come through blood but this thing not mean that blood is affected so need to change the blood;just like this any mechanical problem in the engine can be found by used oil (lubricant) analysis but no meant that that oil is bad for the engine.
Engine Would be various type, rotary or regular type, but every engine must have piston, cylinder, camshaft etc. Which all are moving parts of engine and for this mechanical movement lubricants is must.