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Paul, I've had oil drops come from the valve cover ventilation hose to
the air filter canister and clog the filter on my cars. But this was on
single carb 900 and 99 cars (had one of each).

Don't know if the following is applicable to the turbo models but here
goes: if the valve cover on turbo models has a hose running from the
centre to the airbox ...

... then you could have a partially clogged mesh screen in the valve
cover. You can't see the screen, but if you remove the valve cover (I'll
assume there's some kind of Hall effect distributor that you have to
align to top dead centre before taking the cover off) you'll find the
slot on the inside, leading to the screen, leading to the round hole out
to the hose. When the screen clogs, there will be too much pressure in
the crankcase, causing oil to condense when reaching the space under the
valve cover. The resulting liquid runs down the tube into the airbox.

A good way to clean the screen is to remove the valve cover, invert it
on the garage floor, pour a small amount of petrol into the slot leading
to the screen, let it run through the circular hole out to a bucket,
repeat process until petrol runs freely through slot/screen. Dry off,
reassembly is reverse of disassembly as they say in the Haynes world,
and presto you should have no more oil dripping into your airbox anymore.

Oh yeah - forgot to say - clean the hose leading from the valve cover to
the airbox, petrol is a good idea again.

Tell us how it goes.

/Robert