If you're going to use an engine flush, the Liqui-Moly product to use is their Pro-Line Engine Flush which comes in 500 ml cans (good for up to 5 liters). This is their most concentrated formula. Although the recommended duration is 10 minutes, you can run it up to 20 minutes at idle with no ill effects provided you do NOT raise idle speed.
I've been using it for 9 years with no ill effects, but do not expect a day and night difference simply running it for 20 minutes straight the first time. If the engine is sludged, Liqui-Moly have recommended their
Öl-Schlamm-Spülung for 300 km (188 miles) prior to an oil change. If you use the translate feature on your browser,
this is an interesting and short blog post from Liqui-Moly.
From experience, I would highly suggest replacing the oil filter at the very least, as it does get pretty nasty and turns the oil dark red. Maybe even change the oil with cheap mineral oil, put a decent filter on and run this stuff before changing the oil.