Try a citric acid flush, you'll be suprised at how much gunk is released. Buy citric acid at the pharmacy.
When you refill, put the car up on jackstands or ramps and s l o w l y refill with the breather cracked on the t'stat housing.
This is all laid out in the FSM.

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Democracy dies in darkness, you have to kick at the darkness till it bleeds daylight