[Dave at Pelican Parts]

For some reason, this subject comes up every once in a while... I've saved one of my e-mail responses, and am including it below.

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There are a number of parts you'll need if you want to duplicate the stock setup. The wire holders can be skipped if you simply want the functionality, but you will have to come up with a good way to route the wire.
The stock setup has what I call an "upper plate", which the temp sender screws into with a crush washer. An O-ring goes around this plate. A gasket goes under it, and a "cover plate" below that. Two bolts hold the cover plate (and the upper plate) on, and there is a crush-washer on each of those. On the cover plate is a wire holder which is crimped around the wire, and a rubber boot where the wire goes through the cover plate. Another wire holder is mounted on one of the studs that hold the oil filter flange onto the case. A third is located on one of the oil cooler bracket bolts on the other side of the motor (the wire goes across the lower front of the motor). The wire goes up through a grommeted hole in the engine tin, where it plugs into another wire. That wire goes into the main wiring bundle, where it goes to the gauge.
I highly recommend getting the calibrated aftermarket VDO sender and gauge, rather than the original stock ones. The gauge is slightly smaller, but this is something that can be lived with if you buy the adaptor ring--or make one yourself out of a roll of electrical tape.

Parts:
Cover plate: 039.101.267
O-ring: 021.101.269 A
Gasket: 039.101.287
Small crush washers: N 13.803.2
Upper plate?????: 039.101.275
Cable holder: 039.101.283
Crush washer for sender: 900.123.007.30
Boot: 311.949.149 A
Original sender: 039.919.268
Original gauge: 914.641.118.20
(later version): 914.641.118.30

Aftermarket sender: VDO-323055
Aftermarket gauge: VDO-310012


The part numbers for the stock parts are all VW/Porsche part numbers. The ones for the aftermarket sender and gauge are Pelican's part numbers.
I am not certain that the part labeled "Upper plate?????" is really the upper plate, but there isn't anything else listed that could be it. The notation in the parts manual is "Protective Cap (telethermometer) for oil pan-cover". At first, I thought that was the boot, but the boot is illustrated quite clearly later on. So I'm not really sure, but I can't think of anything else that could be it, and it is in the right part of the book to be the upper plate. Hopefully Thomas or Darren can help you out with that.
Another option is to simply drill the stock plate for the sender (use a nut on the top side to hold it in), but that leaves a good part of the sender exposed to the air which is much cooler than the oil.