flushing it wont necessarily get rid of that "gunk"...slippery/sandy
stuff.
install a coolant filter, buy 2 or 3 filters when you make the purchase...
my 1st filter plugged up in less than 1000 miles, and i might have gotten
2-3k outta my 2nd filter...im on annual filter changes now.
follow the links in my "mods" for vendor information.
You wont believe the amount of sand & crap you'll out of yer cooling system
When I installed the filter (changed it 3-4 times), I ran The OEM coolant for about 5000 miles, then flushed and changed the coolant, the filter and like BP.. change the filter yearly.. whether it need it or not.