This is normal with today's mixed fuel (ethanol and gasoline). Depending on where you live, most gasoline has about 15% ethanol mixed. It is written on the pump. The two compounds are actually quite different as far as their chemical properties. Gasoline if very hydrophobic (repels water), and ethanol has the same inter-molecular forces as water, so they attract one another. So, over time, the ethanol and gasoline will separate and the ethanol will actually attract water. Typically this separation is an issue with boat engines since they sit over the winter months and the fuel separates. Upon startup, this ethanol-water mix (i.e. gunk) will clog up carbeurators and fuel lines. If you look at dry gas, the primary componenet is "methanol" which is only 1 carbon away from "ethanol." If your rig will sit, use some fuel conditioner or even seafoam to help alleviate the gunk and fuel separation issues.