I am running a GX35 clone inverted and have a tube from the rocker cover breather running up level with the top of the crankcase to atmosphere.
When the engine is running oil is carried up the tube from the vent and is lost.
Does anyone know how to overcome this

Should that engine be upright ?sound like the crankcase pressure is pushing the oil out?

Your problem may be that it is a clone and not an original Honda GX35. The Honda 4 stroke trimmer engines can be run in any position while running or being stored and won't leak.I was told this was because of the oil sump in the engine.I had a Troy Bilt 4 stroke on a trimmer and every time I tilted it too far or upside down when using certain attachments it would leak oil. Try to find a real Honda or run it upright. Staton Inc