I have a 90 2x4, 3.9 auto. I use a little oil so I keep regular check on it. I have noticed now that it seems a little overfull and I have a whining noise coming from the truck. The oil seems fine, not milky, doesn't have any gas smell to it, and doesn't burn when held to a flame. Is it possible to get gas leaking into the engine somehow? Where would I need to start looking?

 

Most likely it is coolant from a bad head gasket. Some gas could come in but not large enough.

 

an amount of coolant great enough to see a change in level on the dipstick should result in some milky looking lube oil. This should be visible on the dipstick, but i would also check the lube filler cap. High moisture content sometimes shows up there as water droplets on the filler cap.
if you want to verify presence of coolant or fuel, pick up a lube oil sample kit from a truck dealer (Kenworth, Freightliner etc)

 

I don't believe there's any water or coolant in the oil, like I said the oil looks and feels fine, no milky color, not thin, no discoloration at all. I'm not sure what's going on.