2001 Chevy Express oil in water

I found what looks like oil in my water. I found it when I went to top off the water after seeing the temp guage getting hot.
When I put the water in the res, it had little black dots surrounded by what looked like oxidation, but it was kind of grey not milky.
My check engine light has been coming on and off for a few weeks now and when it does, my little code reader implies multiple misfires.

My thoughts are that it could be a blown head gasket - no water in oil. Yet -
Thermostat - not opening enough and the water is just dirty.
blow-by in the cylinders

But I am no mechanic so that is why I came here.

Can you help a poor man out?

Thanks and God Bless,
Steve