If that's all it is, and it's only a small leak, don't worry about
running out to fix it right this moment - it will cost you big to stop
a small leak.  Just keep a bit closer eye on the oil level, and watch
the morning puddle doesn't get bigger.

 This is something you do the next time you have to change the clutch
or have an automatic transmission rebuilt, because they'll have it all
apart right where you need to fix.  At that point the rear main seal
will be another $50 in labor time and maybe $10 for the seal.

 Just carry around a little metal drip pan and a small plastic jar
with a few pounds/kilos of "Oil Absorbent" (bentonite clay-based cat
box litter), and know where you need to position the pan to catch the
drip.  In case you have to park on someone's nice concrete or brick
driveway (Like at Your Boss' House) that way you don't leave an
"Indelible impression"...

   --<< Bruce >>--