I'm not sure on the 2000 GT's, but with my 2002 GT and my F150 (with a 4.6) 5W20 oil is recommended (not 10W30). This oil is thinner to prevent just such a thing. I'm going to guess that your 2000 GT is the same and you should switch to a 5W20. The Dealer may not be the smartest, but I'm sure they are just following the directions straight from Ford...
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