On the subject of oil coming out of the speedo cable itself which mine suffers from, being a bit bored with watching footie on the tele I took a look at the speedo drive and cable bits and confirmed that the drive bits are in really good condition so looked more closely at the cable. The oil was making its merry way past the fit of the end fitting on the cable sleeve and found that the end fitting was loose on the cable sleeving, odd thinks I.
I have refitted the end fitting onto the cable using a Loctite sealant (black, can't remember the number) in an attempt to block the oil path BUT during reassembly I noticed that the inner drive cable/wire was about 5 mm or so too long relative to the outer sleeve and if I forced the end nuts up I would be putting a tension in the cable's end fittings, hence, thinks I, that was probably why one end fitting had come loose on the cable. So, I Dremelled off 5mm or so off one end the inner wire so that there isn't any tension now on the cable.
Unfortunately, being a new year weekend I can't get any oil for the gearbox to conclude this saga but will update you in due course, as they say...
Iansoady, I guess you've sorted the o ring question you raised by now, but if not, the o ring is in a groove in the part which carries the drive spindle and you need to pull off the outer cover to check/replace the o ring or you will only end up doing a nasty to the drive spindle housing by gripping it with pliers and trying to pull it out, god forbid that you do this without removing the locking screw. Taking the cover off is child's play anyhow and you'll get a good appreciation of the condition of the speedo drive bits.