[aerohead]

The base oil never wears out.
The oil has detergents,dispersants,and viscosity enhancers.These are what wear out at around 7,500-miles under mean,average,USA empirical measurements according to the DOE.
Your engine operates on hydrodynamic lubrication.The parts surf on a film of oil.
You can park a car for two years,pull the cylinder head,and still find a film of oil clinging to your cylinder walls.
Under very extreme,sustained maximum load conditions,it's possible for parts to actually 'touch' one another,which is called asperity,or 'high-pointing'.At this point parts can alternately weld together,then fracture apart,repeating until engine failure.Rev limiters can prevent contemporary cars from ever reaching these loads.
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Synthetic oils begin their lives as a normal mineral oil,which undergoes fancy chemistry to create more stable,longer chain hydrocarbons.
Syn oils have lower viscosity than what they are advertised at.Their pour points are lower,flow better cold,and have very nice high-temp viscosity stamina and long life.Many modern cars come from the factory with syn lubes.
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I'm uncertain about your tailpipe smell,but I'm inclined to think it has to do with the cat also.