That stuff looks nasty. I would flush that stuff out as thoroughly as possible before refilling. I don't know how difficult it is to remove the block drain on a 4.6 (it's not too bad on a 3.0), but that would be your best bet.

Remove the block drain to get as much of the old coolant out as possible. Fill the system with distilled water, bleed and run it with the heat controls on high (fan on low) until warmed up (heat coming from heater inside car).

Drain and repeat. Should be pretty clear by now. There still is some water trapped in the system, so look up the system capacity and add half that amount in pure antifreeze first. Then you can fill the system the rest of the way with distilled water.

If removing the block drain is not feasible, then you'll need to do several more drain/fill sequences before adding antifreeze. On the 3.0, draining the radiator doesn't even drain half the capacity of the system. If the 4.6 is similar, then you may have to do your last fill/drain sequence using antifreeze in order to get enough antifreeze back in the system.

FWIW, I'm in the camp of using BMW antifreeze in my BMW's. I find it's not much more expensive than generic antifreeze and I'm not a chemical or metallurgical expert. I also use Toyota antifreeze in our Toyotas. Anything is going to be better than the crap you have in there now!

AM.

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E70 2010 3.0 M57 AT Titanium Silver with Black Leather
E53 2004 3.0 M54 AT Toledo Blue with Gray Leather
E83 2008 3.0 N52 AT Silver Gray Metallic with Black Vinyl
E46 1999 2.8 M52TU 5MT Black with Tan Leather 250K+ miles (Sold, but not forgotten)