For a start, this model Mazda BT-50 did have a recall to check the transmission’s output shaft speed sensor. If this failed, the transmission could suddenly shift back to first gear which, if it did so at highway speeds, would have been potentially disastrous. But it’s also possible that this sensor could fail and not allow the transmission to shift beyond first gear, so that would be worth a check. A transmission specialist is your best bet here.

Beyond that, simply servicing a component won’t generally fix a fundamental problem. It would be the same with a worn out or internally damaged engine; servicing that engine with fresh oil and spark plugs will not fix the underlying condition.

The automatic transmission is a mysterious, marvellous piece of equipment that manages to work almost in spite of its complexity. Guessing at a problem like this is fraught with danger, so the best advice is to get the vehicle to a transmission specialist, download the fault codes and see if they give the technician any hints as to what’s up.

Transmission problems with this make and model are hardly unknown, so avoid driving the vehicle any farther until it’s been looked at. You may end up doing more damage by continuing to drive it.

The actual problem could be anything from a transmission that is about to catastrophically fail, to one that has simply lost its 'memory' or programming and can be rebooted fairly easily and quickly.

First things first: are you sure the leak is coming from the gearbox and not the engine? It can be very hard to discern the difference in a manual car, although it’s easier in an automatic as the transmission fluid is usually a red colour.

Beyond that, oil can escape from a gearbox through a variety of places. The breather could be allowing fluid out, the pan seal could be leaking, the rear gearbox seal could be faulty or you could even have a cracked transmission casing. Don’t rule out a leaking drain plug, either, as this is a very common source of a leak. You might even find the leak is coming from somewhere within the transmission cooler and its plumbing.