So why does OS do it.

Yes it will. So what? What do you think the oil in your car has in it?

I am not aware of any law, but they have been reusing auto engine oil since the 19th century. And though most of those years most did not even filter the oil! The microscopic metal is not a problem, not near as bad as the dust in the air.

That is not the same thing, no oil other than a small amout of vapor went out that tube. The oil in the crankcase staysbehind and has lot of microscopic debris. Most but not all is filtered, but plenty of unfilterd oil is slung from the crankshaft flying around above the oil pan. Microscopic metal bits are not a big problem, most are very soft. The hard metals have very little wear in the first place. You get much more dust in the engine than the metal in the oil.

I just typed about a thousand words about this subject including everything from PCV valves to oil refining. But I decided to delete it all and say this-

I think the main thing is that people have decided that the benefits of having less mess on a plane is more then the side effect of having an engine get tired sooner.

Myself, I think it would make better cutting or honing oil than a lubricant.-BW

Dust is normally a "soft" particulate that deforms under pressure. e.g. it is mostly organic. Not quite the same thing.

Metal particles on the other hand will do things like score cylinders and rings.

Not too long ago there was an article in one of the RC magazines that talked about the damage done to engines by the bits of burned out glow plug wire that make their way into the crankcase on engines...

Having overhauled a few engines where damage was done by metal particles, it is a bit surprising that the manufacturers elect to re-introduce this stuff into the crankcase.

That said, this SHOULD only be most problematic when the engine is newer, and things are wearing in.

The sandpaper like metal crud I also found in an Evolution engine however keeps me wondering about this practice.
I now tend to remove the backplate and clean/flush out glow engines after their initial 10-20 runs or so.

The dust off your asphalt or concrete or grass strip is mostly clay or sand. Much worse than a few microscopic particles of aluminum.

I thought this was microscopic metal, so the scoring is also microscopic. Dust is much larger than microscopic.

A burned out glow plug is a lot larger and harder than a few flecks of microscopic aluminum.

We are not talking about macro metal particles here, this is micro particles.

Normally it's not.

Clay, sand etc. is fairly heavy relatively speaking. Unless you are flying in a dust storm these things are not flying around.

Most dust consists of cast off cells from plants, people, pollen etc. and relatively light particles that can be carried by low speed air movements.

Of course in a higher wind all bets are off, but most pilots tend to fly in relatively dust free ( the visible variety ) conditions or if not put some sort of filter on their intakes.

The particles of aluminum found at the bottom of a crankcase produces a sludge that can contain particles large enough to create a sandpapery feel. This may mean the particles are larger than sand. Not exactly micro particles.

My worry is those macro particles do damage.

Even the micro aluminum particles can not be all that good for the engine though.

haha you guys are so funny fighting over this.  I think we all can agree that macro particles can do damage.  At what point is micro micro?  Dont you people realize the piston and cylinder walls have a sandpaper like surface on the micro scale????  Look at the surface under a electron microscope and it'll look like the rocky mountains.  You cannot get a truly "smooth" surface.  Even on the atomic level there is friction and wear.  Its all relative.  How much sophistication on reduction of wear and friction compared to the cost of manufacturing and maintenance. 

If micro-particles were truly bad then auto engines wouldn't last for 400k miles.  Sure your engine would probably have less wear if you always used fresh oil, but at what cost of resources and maintenance costs? REGARDLESS this is besides the point.  The question that is being debated is is it ok to reuse the "stuff" that comes out of an RC 4 stroke crank vent.  Will it kill your engine?  Outright no.  Could it reduce its life span?  Possible but can you measure by how much?  Is the cost of reduced engine life greater than the cost of extra cleaning supplies, extra time to clean the plane, and possible oil damage to the airframe?  Who knows.... I think one thing we all can agree on is that IF you want to route the vent back into the engine,,,, put a filter on it to get the macro particles out.  Its a personal preference, not the end of the world. 

To the original poster,,, just do what you want and test the results.  If you dont get the results you want try a new configuration.... this how most things are discovered/invented.