Malibu 2001 Coolant

I have a 2001 Malibu and noticed in the expansion tank that I was growing brown rust sludge in it. I know this stuff can form if the car is only used
for short trips, but I travel to and from work 50 miles each way, so the engine does get properly warmed up.
I also noticed something must have got clogged as the brown stuff is sitting in the engine bay near the engine mount so I am presuming the pressure blew it out the relief pipe at the top of the expansion bottle? Am I guessing right? Do i have the dreaded failed gasket senario? The level in the coolant wasn't low.
I took the expansion tank out and cleaned it all and now the problem is refilling the coolant to the right level. I know I am meant to fill it up to the mark on the bottle where it say 'Full-when cold' but I am having problem seeing the fluid in the bottle as the transparency of the is now opaque. Any yep I bought the GM Dexcool stuff.
Do I fill it to an inch below the expansion bottle neck and this should suffice?
Any help appreciated.
David
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Check the dipstick and the oil cap for any sludge buildup. This would indicate antifreeze getting in the oil. If the level in the overflow isn't going down you probably don't have a leak as long as the radiator is full. GM puts stop leak pellets in the radiator and it shows up as a brown sludge. That is probably what you see. Is the engine and a 3800 that was in for the recall? From what I read the recall put more pellets in the antifreeze. My thoughts are that this helps delay leaks past the warranty period. Then the customer has to pay for the repair...

used
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blew
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It is not normal so you should bring your car in GM for coolant service. Do not add any other coolant other than Dex-cool if your vehicle still (should) have Dex-cool

growing
guessing
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