The one on my series 3 did not unscrew from the speedo, its a sort of clip, difficult to wxplain but easy to do...
First cable I had was all over the place for speed, used to bounce between 40 - 60 when I was doing 49ish never oiled that one.
New one hung it up and ran a mixture of 3 in 1 oil and engine oil down it, smooth as silk now...