Radiator cap
 

I have an 08 wrangler with the 3.8 L bought used starting to see coolant residue around the radiator cap. It has a 15 pound cap on it but I believe from reading it needs an 18 pound cap.
Thoughts?

Factory cap says 18psi. I no longer have the factory radiator but I kept my cap from it. I'd be happy to send it for the price of shipping, but probably easier to just swing by auto store and grab an aftermarket for under $10.

I had the same problem on my 2010 last summer. I picked up a Stant off Amazon and it did the trick.

Stant Radiator Cap - https://a.co/d/ebrJmv0

I first verify your Coolant reservoir fluid level varies when hot and cold. Caps are usually about $12.