What is caught in a PCV Catch Can will vary depending on how well it's designed and how effective it is. There are many Catch Cans on the market, but only a few that perform exceptionally well.
The more effective PCV Catch Cans on the market will trap the most (90%-95%) allowing almost no damaging compounds or oil to blow past.
In cold weather below 50* F, far more water and sludge is caught in a PCV Catch Can.
Some of what is caught:
Water vapor (released during the intense heat and pressure during the combustion process)
Un-burnt fuel (especially in cold weather when cold start enrichment takes place. Gasoline does not burn completely.
Sulfuric Acid (forms when the water vapor mixes with other hydrocarbon based chemicals and is very corrosive to internal parts)
Abrasive soot and carbon particles. These are generally too small for the oil filter to trap, as they are far smaller than the 15 micron size a filter. And if these are not removed by evacuation and flushing, as soon as they enter, these quickly fall and settle into the engine oil causing accelerated wear.
Typical concentration in a Catch Can:
70% water and acids.
23% raw un-burnt fuel.
7% is oil saturated with abrasive particulate matter.
Water, raw fuel, sulfuric acid, other acids, and abrasive particulate matter. If these are not flushed and evacuated (sucked out) as soon as they enter, they quickly settle and mix with the engine oil accumulating in the crankcase.
The abrasive particulate matter is always circulating in the oil. The average oil filter only effectively filters out particulate matter 10-20 microns in size or larger, so smaller particles, responsible on average for app. 70% of all internal engine wear, are not trapped and continue to circulate causing wear to all internal moving parts.