I am looking at this because there is no point in having a load of oil being lost out of the breather.
You probably need a certified one, so I have no clue. I got this:
Haven’t tried it yet, but what can go wrong with an aluminium bottle? The bad point, as they say, is that you effectively recirculate contaminated oil. The vapor leaves, but the separator also condensate the “bad stuff” from the blow by gases. I have a hard time figuring out what exactly that “bad stuff” is supposed to be. In the crankcase the “good stuff” and “bad stuff” mixes all around in any case, also without a separator. Rotax use an external oil tank. The crank vent also go there, which effectively makes the whole oil tank a “separator”, no ill effects there. The oil is changed every 50h in any case.
A separator allows you to have more oil in the engine on average, considerably more without loosing any. The oil continuously gets contaminated and degraded due to blow by gases. With more oil, this contamination is diluted more, so the effect is in fact cleaner oil IMO.
The bad point, as they say, is that you effectively recirculate contaminated oil.
This is the bit I don’t understand.
Wit a normal Lyco-type engine, you have a vigorously agitated air/oil vapour in the crankcase. There is also a net flow into the crankcase due to blow-by past the piston rings. So you have a net flow out of the breather pipe. This carries away some oil vapour, so it carries away oil.
In addition to any net flow, the oscillatory nature of the agitation will cause a bit of the vapour to appear just outside the breather outlet and that little “packet” of gas will get taken away by the slipstream. Hence the positioning of the breather outlet makes some difference to the oil loss of an engine.
Hence oil on the belly of most planes.
Now let’s say the engine loses 1 litre every 10hrs and you run a 30hr service interval and the sump holds 10 litres. So over the 30hrs you will have lost 3 litres which were topped up. Now, you will probably not top up right before the service 
so you have done 2 topups. So the dirty oil gets diluted with 20% of fresh oil.
I am sure somebody who can do maths can work it out exactly but intuition tells me this is going to be a logarithmic (exponential) relationship because quite obviously no matter how much oil the engine loses, no amount of topup will ever remove every last bit of the dirty oil.
And the engine cannot possibly care between running with oil with a given amount of crap in it, and oil with 20% less crap. The result is similar to running for 30hrs versus running for 36hrs. It cannot make a difference!
So I think that old story about oil separators being bad because they feed back dirty oil is nonsense.
Well, unless your engine is losing a litre every hour in which case you will be putting in 30 litres of topups and then the engine will for sure have nearly fresh oil all the time 
Can anyone see a problem with this reasoning?
An oil separator works by cooling down the air/oil/water mix and thereby condensing the oil and feeding it back to the crankcase. Depending on the operating temperature of the separator (which seems to be mostly dependent on its exact position), it can also condense water vapor and feed that back, thereby causing corrosion. Water is a byproduct of combustion and plenty of it is found in the blow-by.
I know there were cases of severe engine corrosion caused by an oil separator. Most mechanics I talk to strongly discourage the use of oil separators but that is typically not based on personal experience. Also they don’t have to clean aircraft bellies…
One simple way to save oil without any additional devices is to keep the level close to the minimum. I used to top up my O-360 to 8 quarts and wait till it drops back to 6. Now that I only top up to 6.5 or so, the average oil consumption has dropped roughly by a factor of two. (Incidentally, the 6-quart minimum is listed in the aircraft manual, while Lycoming says the safe minimum is only 2 quarts).
Ultranomad wrote:
(Incidentally, the 6-quart minimum is listed in the aircraft manual, while Lycoming says the safe minimum is only 2 quarts).
Isn’t the difference that the aircraft POH gives a figure that will ensure that the amount of oil will not drop below 2 qts during a maximum endurance flight with the maximum accepted oil consumption? Also the oil dipstick may not measure quantities lower than 4 qt, say.
Exactly. My IO-550 consumes 1 liter per hour between 8-6 qts …. and only 0.2 per hour if i take off with 6 ,..
For the reasons Achim already mentioned i did not install it, i find the corrosion theory plausible.
Ultranomad wrote:
One simple way to save oil without any additional devices is to keep the level close to the minimum.
Yes, indeed. However, it feels so much better to not depart on a 6h flight over open waters with minimum oil level…
My engine can hold 9qt (8qt in sump, 1qt in hoses, oil cooler, filter, etc.) but the max it will hold on the dipstick is 5qt. Once it is at that level, it doesn’t really burn/lose any oil, I never add oil between my 25h oil changes. If I did not have the oil mess on the aircraft belly and could keep more oil in the engine, I would be very happy. Thus my interest in a separator.
Airborne_Again wrote:
Isn’t the difference that the aircraft POH gives a figure that will ensure that the amount of oil will not drop below 2 qts during a maximum endurance flight with the maximum accepted oil consumption?
Exactly and therefore big fuel capacity = big oil sump capacity. A “healthy” engine is allowed to burn 0.5qt per hour or sometimes even more. That’s more than 2-stroke engines…
An oil separator works by cooling down the air/oil/water mix and thereby condensing the oil and feeding it back to the crankcase. Depending on the operating temperature of the separator (which seems to be mostly dependent on its exact position), it can also condense water vapor and feed that back, thereby causing corrosion.
OK; that is plausible.
However that would render separators mostly useless, due to the OAT variation around the year, altitude, etc.
One could have a temperature controlled separator Well, heating it would be easy, but not cooling it. What is the ideal temperature?
Water boils (completely and instantly) at +100C; less at altitude. Oil boils at some very high temp. But one doesn’t need engine oil at +100C in order to boil off the water condensed into it; that much we do know. Typical oil temp is some +170F which is +77C and water would disappear pretty quick at that temp.
So it seems that a separator needs to be maintained at a fairly high temp, but the margin might be quite small.
One has to make sure the temperature inside the separator is always above 100°C. The Air Wolf instructions mention that a remote temperature probe should be temporarily installed to verify the temperature in flight. I believe if one follows that procedure and makes sure it will never be below 100°C, a separator can be a good thing and I agree with Peter’s view that the negative effect of feeding back dirt is negligible for somebody that does frequent oil changes.
