Oil tank breather

Mark, I'm root causing a way too high oil consumption after a service overhaul. Oil tubes are replaced among other things. There are no obvious reason that  should cause high oil consumption besides the breather pipe from the oil tank  was partly collapsed. The aircraft haven't been in the air yet so I don't know if it is fixed. How much oil did your engine consume per hour? Did it come back to normal after fixing the breather?

There may be a misunderstanding of the Oil Circuit.

The Oil Feed Hose from the Oil Tank to the Oil Pump is a suction line and if restricted, "May" collapse when Hot and Soft.
The Oil Return Hose from bottom of the crankase is under slight pressure (~2-5 psi) and May expand  When Hot.

The Oil Beather Hose is actually a "Vent" Hose, there is no  "Breathing" going on here.
The flow is always one-way from the Oil Tank to the end of the hose.  There is No "Inhale".
The pressure in this hose is well less than 2-5 psi in the Oil Return Hose because it is 99.9% air flow and does not build any back pressure pushing the large quantities of Oil that the Return Hose does.
Any Restriction in the Vent Hose is going to raise the pressure and not collapse the hose.
If you were loosing Oil Out of the Vent Hose, the end of it would be drippy wet with oil and you would likely see a puddle on  the ground under the plane.

Tie-Wrap a Solo Cup to the end of the Oil Vent hose.
If after an appropriate time you find an inch of Oil in the Cup, That would be an issue.
If it only accumulates a slight Oil film from the vapors, start looking elsewhere.
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Are you overfilling the Oil Tank???
Before the first start of the Day, "Burp" the Oil Tank,  and then do it again.
Do not over fill or it may get vented overboard.
Burping a Hot engine may give a False low indication because the Oil has not had time to drain from the far reaches of the engine. 

Peter, the oil usage has been 100 to 150 ml / hr.

I do not yet have a definitive answer whether the replaced breather pipe has solved the problem.

Bill, my initial post was a a little vague.

The installation had a catch tank to which the vent hose was attached. A looped section of hose from the side of the catch tank was routed to the bottom of the firewall (with a notch as per the Roatx Installation Manual) . It was this looped section which was kinked and therefore causing a restriction. When the looped section of hose was hot it folded completely blocking the vent hose.

The question is, since the Rotax uses crankcase pressure to return the oil to the tank and if the vent hose is blocked, as explained above, is the 2-5/6 psi crankcase pressure still sufficient to return most of the oil to the tank? Or will, in what is now effectively a closed system, a significant amount of oil still remain in the crankcase / engine and make it's way past the rings or valve guides?

This type of oil consumption occurs in some automotive engines if the crankcase breather is partially or completely blocked. I am trying to ascertain whether this can occur with a Rotax 912.

The catch tank has about a 1/4" of oil in it after 10 hours.

The oil level is monitored by 'burping' the engine twice before the first start of the day and is topped up based on the dipstick level.

If you were to block or mostly block  the Oil tank Vent Hose, the pressure in the Oil tank would quickly come to near equalibrium with the crankcase pressure.
With no, or very little, pressure difference there would be no reason for the Oil to return to the Oil Tank.
I can see where the oil level in the crankase would soon reach the level where the crank shaft would start splashing Oil from the bottom of the crankcase onto the bottom of the pistons only to be drawn past the rings on the intake strokes.

I have avoided using the term "Oil Pan" because where a typical automotive engine has a few inches of clearance below the Crankshaft for the the Oil to collect, The 912 engines have very little volume below the crank and assumes the bottom of the crankcase is being constantly scavaged and contains very little oil at any particular time.

So YES, I would agree that a blocked Oil Vent hose could cause excessive Oil consumption.
The elevated crankcase pressure could also force Oil past the Valve Guide into the Intake and Exhaust manifolds.  (Good Observation!)
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Since you did not mention it, I assume you never lost oil pressure;  Indication that Sufficient Oil was being returned to the Oil Tank to prevent the Oil Pump from drawing air.
Your vent blockage may have been bad, but not total.
I could see that in a total blockage the crankcase pressure would start to approach combustion pressures and at some point one of the hose connections might self disconnect.

I bought a Remos with a recently overhauled engine with a big bore set up from Edge. The engine had been sitting for serveral years . After 20 hours the prop seal began leaking then the rear crank seal went. We plugged all those leaks but like Mark it uses half a quart or more every 4 hours . I put a bottle on the vent tube and gets only a small amount of oil in the bottle. All compressions are 80 plus and never a drop on the floor. Where is it going?

My Kitfox uses almost zero oil with 900 hours on it.

I don't use more than 10% 100LL is it alright to switch to a less expensive oil? I use  the sportplus 4 but its $13  a quart for some reason.