Does anybody know the part number or correct size for the seal/gasket that sits on the valve cover around the oil filler hole. Took my oil cap off earlier and the gasket disintegrated. 

You can only get the seal with a new cap.   

Dodge # 05014704AB

Stant #10111

Gates #31118

Any part store carries the replacement oil cap. I've got a start oil cap and it does not have the ratchet cap.

I'm confused on the gasket part  which is nothing new for me, happens quite easily. I thought it's just an oring on the oil cap, did some come with gaskets ? 

 

Yea it’s not the o-ring, and I would have never realized it had a gasket there if it hadn’t peeled off when I removed the cap. 

Weird, I'd like to see a picture

3936876 is the only part I saw even close to this and came in oil filler tube kits for 12v’s and C series engines. Thickness of construction paper.

I looked at mine, it's an oring. Your cap may not be factory. Wonder if you couldn't just use an oring instead of gasket. My oring seals inside the hole in your case it would get sandwiched between cap and top of the cover like gasket. Just don't over tighten it, snug. 

You might be right, it very well could be an aftermarket cap, but my cap has an o-ring, the gasket in the pic was around the filler hole on the valve cover. The truck is about to get loaded up on a flatbed and taken in for a new transmission. I’ll add this mystery to my list of to do’s for when I get it back.

Genos garage has caps for a good price. I actually just got one from them too.