It's an undertray, a sump guard would be heavy metal and rigidly mounted to the chassis.
In the ETKA diagram above, the short undertray is the one for the petrol engined cars (apart from the LCR) and the long one is for the diesels. The diesel undertray has a secondary sound-deadening function.
Undertrays improve the car's aerodynamics, smoothing airflow under the car and helping to ensure that the engine bay is a low-pressure zone, promoting airflow through the radiators. They also keep stones and water from damaging the car's electrics and the low-mounted boost pipes on turbo cars. Well worth having, I think.
The LCR has a special undertray, full length and with NACA ducts. The TDI 150 also has a special undertray, as I've just found out - it seems to be thinner than the ordinary TDI undertray, presumably because the 150 Sport is 15mm
lower than other Toledos, and to have a dished area below the turbo.
Comparative pictures here.
TDI 150 undertray. This one is damaged (previous owner hit a rock from the look of it, ripping off the back edge and some of the side mounting points). The oil is from the boost pipes, which get it from the PCV system, recirculating crankcase vapours (which are of course oily) into the inlet system.
TDI 110 undertray.
As you can see, the diesel undertrays don't have a hole under the engine sump, despite what the ETKA picture seems to show.
Get one from any dealer, parts supplier such as GSF or from David at Sere motors, one of the forum sponsors - see the sections below.