Oil in the expansion tank

[Transporter]

What would be the active ingredient in the radiator flush that would be able to get off all the oil?

Grease and oil is very acidic. Degreaser is highly alkaline. It works by neutralising the grease and oil causing it to dissociate/fall apart and it then becomes easily taken up by towels or flushed away. If a radiator flush is able to perform the same job then it must also be highly alkaline.

Your method with the radiator flush may work but I would be looking at the chemistry that makes it so effective. By far the most logical way to do this is the way described by cetane and i guarantee it is also the cheapest. The Wurth product you posted gives you the pH level but notice how it says it needs to be flushed away straight away in bold letters. It's applying the exact same principle of neutralising the buildup and breaking the bond. Either method if done correctly should cause no harm.

Still, the radiator flush is right product for the job and not a degreaser which you don't now what other chemicals are in it and what other effect it will have on cooling system.
It is modern engine cooling system we talking about not a 1970 farmers tractor on its last legs.
You should also remove expansion reservoir and clean it, before you start with flushing.

I also use 70mm dia. and 500mm long clear PVC tube on adapter on the top of reservoir for easy removing of surfaced oil via vacuum extractor while the engine is running.

[Tornado T5]

Just got a call from my dealer telling me that my oil cooler needs to be replaced under a VW campaign...so it's definately a known issue with an official replacement program for T5's..mines an MY07 built in June '07.