Rust inside oil cap?

[Coastie]

2003 Dodge Ram 1500 w/ 5.7

My truck has 53k miles on it. For quite a while (20k miles or more) there has been what looks like dry rust on the inside of the plastic oil cap. I was not really concerned with it until today when a coworker mentioned that it was not normal. We checked the other nearby vehicles and none exhibited the same rust colored deposits.
Doing some searching on the net revealed that the rust colored deposits can be a normal occurrence due to ambient humidity, but there seems to be no logical proof to that, or what the cause would be. Metal oxidation causes rust, so it would seem this could potentially be a metal (internal engine) problem. The truck has spent significant time up and down the eastern seaboard from Florida to New York. I have not done a used oil analysis on this vehicle, and the oil was changed just a few hundred miles ago. (used mobile 1 for the last 30k or so) With the recent concern over the mysterious rust deposits on the plastic oil cap, a used oil analysis may be done at the next change interval. Any info to help solve this mystery would be appreciated.

This is the inside of the plastic oil cap

This is looking down into the oil fill port

 

[Blue99]

You can relax, it's not rust. It is dried out condensation from the colder, winter months.

The oil fill cap tends to become a "cooling tower" where hot gases from the crankcase condense on the inside of the plastic cap which is cooled by the outside ambient air.

Typically, people will report a gelatin like substance, or "mayonaise" deposited inside the cap. It is a combination of combustion by-products, fuel and moisture. Many times it will have an orange to red tint.

You may want to check/replace your PCV valve, just to insure the system is functioning correctly.

 

[NYEngineer]

Blue99 is exactly right. I used to see this all the time on outdoor standby generators.