In a moment of stupidity I poured a bit of engine coolant additive in the engine oil thinking it was engine oil bottle without reading the label (stupid me)
I have a very Touareg.
What are the implications? Do I need to flush it all?
500ml isn't a lot - but it's enough to cause problems.
Assuming it's a glycol-based coolant it can rather seriously affect the oil's performance increasing viscosity and reducing lubrication - potentially even causing blockages in the oil filter.
What to do next depends on whether you've started the engine since you added the coolant, if you haven't you can get away with "just" dropping the oil and replacing it. If you have I'd do a full oil + filter change.