Oil return line

12mm (~1/2")(-8AN) will be fine.
10mm(~3/8")(-6AN) will be a problem.

8AN is roughly 1.8 times the cross-section of 6AN hose.
That is a Significant difference.

The problem is  that the rotax fitting size is only 10 ID?

Is there any advantage to going with the teflon?

Fred Stated,
The problem is that the Rotax fitting size is only 10 ID? ...

ALL fittings are going to have an ID that is smaller than the hose that slips over them.
The problem will not be the smaller diameter for the few centimeters of the fitting. 
The problem will be in using a couple of Meters of hose of the smaller diameter.

Yes the remos with the big bore. I have now replaced the main seal twice and the prop seal once.

To trouble shoot leading edge set me up with new larger oil return line hose and fittings for about 250 bucks. I also put a larger size vent hose on. the return line is only about 19 inches long with no sharp bends.

Our first check of the pressure at full throttle was 6.5 psi which is high but at the top of   the limits.  Not sure what else can be done to lower the pressure?

I did set up a new liquid damping gauge to see if I get any different reading

Rotax wizard

I have 3 different crankcase pressure max figures from rotax documents. The one you sent me 8.7 psi

one from this attachment  (4,35 psi or 8,86 in Hg).

Then somewhere else I can't find right now is says 6.53 psi

Which one is correct?