[Vwdiesel] A2 speedo cable gasket

[Scott Kair]

    Awhile back I changed the transmission oil in my 92 Jetta TD, swapping
in Redline MTL for the original goop.  I refilled through the speedo cable
hole, as per usual procedure.
    After about a week, a small stain began appearing on the garage floor.
It was transmission oil, but with the belly pan diverting drips, it took
awhile to discern the source.  The torx head bolt wasn't maintaining a grip,
so I tightened it a couple of times, but the leak came back.
    Yesterday, I pulled the cable and found that the rubber/composite gasket
that fits between the cable mounting plate and the transmission was pretty
well trashed, which would explain the leak being evident behind the cable
hole.  The parts guy at the dealer told me that I'd have to pop over $50 for
a new cable assembly just to get the gasket.  Right.
    Has anyone tried to seal a speedo hole with non-hardening silicone
gasket maker?  Any obvious pitfalls to avoid?
    BTW, when I pulled the cable, the oil was up near the top of the hole.
In order to access the drain hole, I had to jack the car up and pull the
right front wheel.  I suspect that the old oil didn't all drain out.  Is it
possible that I overfilled the gearbox with the two quarts of Redline?
    TIA,
    Scott Kair