Thanks for the reply. The reason I was asking is because I have gearbox whine following bearing failure whilst on a driving holiday to France/Portugal - I carried on driving for a further 3,000 miles after the problem first arose. I replaced the mineral oil put in when all the six bearings were replaced with Liqui-Moly SAE 75 W-90 fully synthetic (which I had used before on my old 156) which reduced the whine quite a bit but not totally, I thought that maybe some of the old mineral oil may still be in the differential area of the gearbox. The mechanic that changed the oil did not have a suitable allen key for that higher drain plug.
I guess I'll just have to live with the whine. Trouble is that once you know there is a problem you listen out for it.