What would be causing oil like substance in my water resivoir of my Ford F-150?

I have an oily substance in my water resivoir. What would be causing that?

That's the coolant reservoir… NOT a water reservoir. It's common to get a little "oily" residue in the reservoir. It may be from slight head gasket leakage or even old antifreeze sludge. I'd have the cooling system flushed and new antifreeze put in. If the motor does not overheat or run bad I'd just drive it after flushing the system.

Guesses would be a blown head gasket or leaky valve seals since they're the most common place on an engine where the coolant and any lubricant would be able to mix.

Explain in more detail please.