The thick, black stuff is probably mostly sludge?

Another way of flushing is to use a cheap mineral oil as the flushing oil for maybe 10~15 mins, after draining out the used engine oil. Then drain again, change the oil filter and fill up with the engine oil of yr choice.

U can also shorten the oil-change interval to maybe 2k km or so as a flushing job, then change oil again and follow the usual oil-change interval.

Some people just use diesel engine oil that's also rated for petrol engines, as diesel lubes usually contain more detergents compared to petrol engine lubes. Examples include Caltex Delo 400 15W-40, Mobil Delvac MX.

Don't think auto-rx is easily available here. And I was told it takes like 10k km to work properly.

I think the STP Engine Flush states on its label that it's not corrosive. Donno whether this holds true for engine flush from other brands though.