I'm up to 18,000km (11,000 miles) on my first chain.   I'm finding as I check the tension it runs anywere from 10mm to 40mm on the low/high side.   A fellow motorcyclist said I could lube it out in order to get out the tight spot, or should I just get myself a new chain and sprocket?

I don't think it will "lube out".  New chain is needed.  Sprocket is debatable, it depends on the condition, and how well it was taken care of. Cheap insurance to just replace it.

Lube it out?  Never heard of it.  Chain drive motorcycles have been around for 90+ years, and I would think some of us who have been riding 20+ would have heard of something like that if it worked.

At 18,000 miles, it sounds more like it's time for a new chain (and sprockets, both of 'em).  You got some pretty good mileage out of that chain, I'd say.

And, the fellow motocyclist may have thought you meant a binding link, which is different from a tight spot on the chain.

Tight and loose spots come from unevenly worn parts of the chain and/or sprockets. Lube won't do a thing. Thank them for their long service and replace the lot.