2nd vote for boat fogging oil. When ready to reinstall add 1/2 cup fresh gas. Shake. Dump. Install and fill.
I did a little research on fogging oil. I did not find anybody specifically using it for gas tanks.
Fogging Oil seems to be mostly used in top ends by taking out the spark plug and spraying it in there. "Not too much" for that seems to be the watchword (albeit there seems to be little information on how much to use.)
Various problems like not being able to idle and smoking are cited. Relatively speaking it seems to be non-compressible. I'm wondering how much to add and how I get it to coat all surfaces of a gas tank, particularly a KLR650 gas tank. I can't see in there and am not likely to invest in tools that would let me see in there. It's likely to settle into the narrowest of spaces of the gas tank-so how soluble is it after a length of time (though I acknowledge that might not present a problem)?
I've never used fogging oil.
So, a liquid that I can slosh around to be reasonably sure all is coated seems indicated. Motor oil for this purpose is cheap.
Cost. 10-20$ is probably my budget. 25% of a new-to-me but used tank would seem to be going a bit far as future use for me is as yet undetermined. I just know the tank, in great condition now, is going to get a bit of beating if I use it. The tank I have from a parts bike, and that I will put on, will find any new scars to be unremarkable. Considering what I paid for the parts bike and what I've already used, the cost of the replacement tank is negligible.
My consideration is cleaning the bike up a bit before it gets sold someday though even that is suspect--my other rideable bike is 18 years old and I've known it since the showroom floor.