So I'm 4300 miles into a 8000 mile ride. I look at my rear wheel after finishing up day 12 and there's gear oil on the wheel. It wasn't there that morning. In the past two days I've been in temperatures between 35F and 95F and elevations between 500 and 10,500 feet. Lots of sweeping turns down 6% grades, too.

The level is now 1/4" below the filler hole. How concerned should I be? Did it just vent some excess oil?

Keep an eye on the level; at this point there is nothing to worry about but I'd be worried if it keeps loosing oil.

I've found that 80W140 pukes out a bit but 80W90 never does.

If it continues, you are probably looking at replacing the big (5" diameter) seal that goes around the hub drive.  Not an overly hard job, but you do have to pull the pumpkin off to do it.  Some have had success by running a piece of film or thin cardboard to clean the seals groove much like the forks can be done.  I've never had that luck.  Short term, just keep checking and adding fluid until you get home.  Don't tighten that filler cap too much, it is cast aluminum and a few have broken theirs rather easily.

You may want to find a 5 minute car wash every so often if it is creeping onto the tread.  That kind of oil will take you down in a heart beat.

EDIT: Spoke too soon... it started weeping a little oil on the way home from work tonight.    
Just ordered the replacement seal at Ron Ayers.

So disregard the post below.

I'm going to tell you how I fixed a similar problem on Sunday... and it's going to make you cringe!  (but it worked)

When I changed my rear tire, I had the final drive off for cleaning and lubing the splines.  I must have pushed some dirt/dust into the seal while I was cleaning, because when I came  back from my test ride there was gear oil splashed all over the wheel... and it was dripping on the tire once I parked the bike.

So off came the wheel, and the final drive again.    It was leaking around the seal.   

Here's what I did.  Now mind you, I figured I had nothing to lose: I didn't have a new seal on hand, and this one had to come out if I couldn't remedy the leak.  I drained the final drive, put it on my bench, turned the regulator on my compressor down to almost nothing... then gently blew a few short puffs of air into the vent hole while holding my hand over the seal.  I could feel the seal bulge a little, and some oil seeped out between the seal and the drive.

Obviously this is a big gamble because you may end up pushing the input seal out... but I think the key is to use just a little air, then let the pressure bleed off.

I saw a couple drips on the wheel today in 150 miles. I might be residual spooge. The level still looks OK. It has to hold together for another 3000+ miles.

Yeah, I have a leaker. I'm losing about an ounce per 1000 miles. Sure makes a mess...