Muffin, not sure, but i would first off give the bike a good wash & degrease, then dry it off then take it for a ride far enough, to get motor & oil to full opperating temps. say 20k ride if possible, then on return sprinkle baby powerder over the area the oil is see what you find. the BP will stick to where oil is should lead too where its leaking. rocker box is common on evo's. anyway try that.

 Just a thought Muffin,

had a mate with a similar problem on his Evo, drove him nuts for weeks. Nothing seemed to be wrong, he even checked the front brake system (dot 5), just in case there was a leak there. Ended up being loose rocker box screws on the rear cylinder. 

Cheers,

Cris

 clean & dry bike motor  then puff talc powder over where you think it's  leaking then go for a little ride  it will show up straight away just look for the wet spot ,    i could say something dirtyminded about this like if you can't find it ,  sprinkle talc powder on her & look for the wet spot & away you go 

Guy  Rules ,1 past stripping then 2back