Stabilant 22 (and 22a) is a brilliant product for dealing with corrosion between connector terminals. Typical uses are in automotive EFI wiring harness to sensor/injector/actuator connections, particularly where a little extra variable resistance can wreak havoc with the data the ECU is receiving. The resistance is typically variable as the connector is in a fairly extreme and variable environment. i.e. lots of vibration, temperature fluctuations, humidity fluctuations etc. Add a little corrosion into the mix and the electrical properties of the connection begin to resemble mud.
S22 is almost like a cross between WD40 and di-electric grease. Penetrate, corrosion resistance, and friendly electrical properties.
It has a big following with owners of early EFIrunning Bosch systems where the manufacturer opted for the cheapest possible components. Think Ferrari 348, 355... These cars generally had engines tuned to extract as much power as possible from the engine, so the margin of error that is acceptable in sensor readings is less than that in less stressed engines.
A 355 that is throwing all sorts of ECU codes, running like a pig, showing intermittent misfires and other oddities can be transformed in a matter of 15 minutes by disconnecting all ECU connectors, cleaning away traces of visible corrosion,applying a drop of S22 and re-plugging everything.
The 'long term' fix in these cars is to replace the entire wiring harness with one made of better quality cable, gold plated connectors and proper heat protection. S22 is the cheapest way to 'prove' that this is needed.
Yes - Ferrari chose to make their cars out of the cheapest junk available, while Mercedes Benz of the same era used much better quality components.
Having said all that, S22 is only going to help where there is a connection with actual corrosion, even if it is not visible. It is also only a temporary fix as it eventually will get displaced and corrosion will resume. Di-electric grease is much better at preventing corrosion in the first place. Of course, best of all is to not have a connection or to seal the whole thing in a big blob of epoxy.
In summary, S22 is a really useful diagnostic tool, but not much of a long term solution.
I should also mention - S22 is available from VW dealers here in the USA. Find an online VW parts store attached to a dealership and you'll get the best prices.
As goo das it is, I don't think it'll help your diesel VW run any cleaner