Why does it look at dirty and oily inside the throtle body and into the intake manifold when you take off the elbow?

On higher milage cars I mean.

 

Happens to any car ever made with a PCV system.

The system vents the extra pressure from the crankcase into your throttle body. The vent line has baffles in it to keep oil out, but they don't catch 100% of it, so a film of oil that got through the lines builds up inside.

 

how often should it be cleaned? seems like that gunk in there cant be good.

and is there a way to prevent it? or is that bad?

 

There's no real way to prevent it, minus removing the PCV system, which would be silly. Once a year is fine to clean it, it doesn't generally cause problems.

 

it is just combustion and that is normal for just about any car these days. run higher octane fuels. Also, run seafoam or G.M combustion cleaner and you will see a big difference in fuel managment.