I am assuming you have a "X" series car. (if you've got a single rail) The oil around that valve could be coming from your rocker cover gasket. On these, the cork gasket goes hard or the rocker cover works loose, the oil can then drip down the back of the motor and settle on the top of the gearbox.
Hope it helps
476 EF XR6 Wagons - 198 were Manual
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