The o ring on the drain plug has shit itself!! Probably  a stupid question but will any o ring do or are they treated for heat? 

Silicon vs. rubber orings would be my only thought.

-0-13 Viton O rings from your local gas/hydraulic/plumbing/bearing joint

Buy them in "bulk" , I get 100 at a time. Works out a fraction of the cost of "genuine" HD ones from your Harley dealer , they charge $1.35 a piece !

I use ones from a $2.99 set from a junk shop. Just a new one every time.

O-Rings (engine case, tranny pan and primary case):

The o-ring in the fuel quick disconnect on the fuel tank is also the same size as the drain plug o-rings

john deere have an equivalent o ring,dont know if the price compares but handy for me [ work as tech at a dealer ].have the number some where.cheers 

It's the oil drain plug 

Any bearing store or industrial store should stock them

The fuel line O-rings are not the same as the drain plug O-rings.  The fuel line O-rings are:

Fuel Line Coupling O-Rings:

got curious..went and measured the factory drain plug o ring[ fxdl '07],that close to the same size as jd o ring its not worth measuring.jd o ring suits fuel connectionas well.

Blokes are we getting a little pedantic here. What fits and don"t leak is the go.seriously.

In my case for a 2008 dyna it is the same size or close enough to the same size to do the job perfectly. 0.002’ difference on the thickness ain’t going to matter. That o-ring has been in place on my bike for about 3 years and is doing its job fine. They are o-rings, they give a bit..

I picked one up today from the ag dealer ??