Last September I took my 2012 Silverado Duramax to the dealer where I bought it. It was due for engine oil and filter and fuel filter before we pulled the camper on a trip to the Grand Canyon. Checking the engine oil in KS or CO I notice it's pretty dark already... dark enough you can't see through it and make me wonder about the dealer. Is this normal or did they not let it drain completely or ??? I told them to change oil AND filter.