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lubes
When an engine is running,it's lubrication is functioning on a hydrodynamic basis.Metals are separated by oil film,which ski on a continuous wedge of lube trapped in between.
During "jerk",chugging,and destructive detonation,loads can reach levels where the lube is completely squeezed out of the void leading to "high-point" or "asperity",where metal-to-metal contact actually occurs.
Unless you abuse a vehicle,this isn't an issue.
And if a vehicle is operated on a regular basis,"drain-back" isn't an issue either.
I have disassembled engines which have sat for years,and all internal parts were still liberally coated with oil.
Reducing viscosity is the only way I have ever heard of which can effect mpg.Switching to a full synthetic like advanced Mobil-1 would do this,as it's viscosity is like a 0W- weight oil at start-up,but certification criteria forbid the oilmaker from stating it in advertising.
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