Update: The sludge in the coolant tank was caused by the "engine oil cooler". There was a leak inside it, which lead to the oil/coolant to mix in the coolant tank and produce a tan sludge. I had the engine oil cooler replaced, and it solved it. The place I went to had a $100 labor rate, and the cooler itself cost about $150 when bought by the shop. I get a discount on labor so it's $80/hr, and therefore the total that I paid was $503. This may run a bit higher depending on shop rates, but it's definitely a good thing to check out if you notice buildup in the tank!