[super vw]

you could remmove the rear (rear is rear) engine tin to let the water and crud escape. It's fairly easy to remove... check out a repair manual if you have never done it.
You can use any detergent you like, just make sure its not going to harm the paint or the aluminum (some acid based cleaners can corrode things).
A controled stream of water works best for me. you could use high pressure... but be carfull! you could end up taking more off than you planed. i like garden hose pressure for the engine if its not to thick of a buildup.

Take care to cover all electonics (like the distributor...ect) aluminum foil works good for this, that way it wont hose off and you can see it easy.

Jonathan