there are two block drains - one for each side of the block - its usually a 14mm head bolt, That sludge you see is due to air in the cooling system.

If you notice - the radiator cap is a bit lower than the Z shaped steel pipe on top of the lower manifold - thats where it locks in air and dexcool starts to accumulate this sludgy junk. I found that its best to backfill the engine with coolant slowly from the heater hose that leads to the back of the thermostat, then fill with a big funnel stuck in the radiator cap opening and keep the level higher than the entire engine start and bleed through the bleed valves.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)