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Since the blower was just rebuilt lets blame that.
I'm assuming crank case pressure was normal before the blower rebuild.
The blower gears are lubricated by engine oil, the oil drains back to the crank case.
There is not supposed to be any "boost" pressure from the blower getting into that part of the blower where the oil just gravity drains back into crankcase, but the fugitive air pressure follows that oil drain and over pressures the crank case. Something is wrong inside that blower letting the pressurized air get to areas it isn't supposed to go. Oil seals, end plates, something not sealing correctly. Take it back to the rebuilder.
Does this engine idle for long periods? The reason I ask is that oil coming from the air box drains can be considered normal. A very small amount of oil can be scraped from the cylinder walls and enter the air box through the liner ports. It can be hard to grasp because the air box is full of positive air pressure...right?
But just like in any powerful river or stream, there are what are known as "eddy" currents or eddies. This is where there is a bend in the river and a small amount of the flow can get caught in these eddies and, sometimes, flow backwards. The same thing is taking place in the air box.
During higher RPM operation, the air box is fully charged and not much eddy effect is present. But, when the engine is backed down to idle or if the engine is allowed to idle for long periods, then these eddy currents are present and oil can enter the box. This is known as "oil slobber". Slobber is a big reason for having the air box drains and why the drains MUST be kept clear at all times.
While undue amounts of oil in the air box COULD be attributed to leaking blower seals, I would hope that there was some discussion about oil slobber and a visual inspection of the blower rotors and seals before the work was authorized.
Sounds normal. I don't think we get that oil here in the States but it sounds perfect.
I'm going to say yes, I think that's a possibility. If the mounting gasket had a flaw at or near the rotor bearing oil drain back holes then I suppose the crankcase could be pressurized in that way. Having solved the issue by simply removing and replacing the blower tends to confirm that something like that had to have been the cause.
Glad to see you got it sorted out.