All the oil changes in the world won't stop a leak if your rocker box covers are warped.
And if there's no warpage, it may just need new gaskets. Those could be original to the bike and are simply worn out.
It is very common on Evos. Synthetic/dino oil has nothing to do with it. I've had synthetic in mine for three years now and it hasn't leaked a drop..... since I fixed the warped rocker boxes that is.
There's a thread about this on here somewhere.
Take off the top cover and place it on a flat surface. I mean either plate glass or a granite countertop. If it rocks back and forth at all between diagonal corners, it's warped. Use some sandpaper on the countertop, and lay the rocker cover on it and move it in a figure 8 pattern gently until it's flat. You'll also remove the chrome from the gasket mating surface--a good thing. check the middle piece as well but you can get away with a little rocking on it. Doubt it's a problem
Once the tops are flat, get a new gasket set and put it all back together.
There's a slight chance that it could be the bottom rocker box gasket--between the lower rocker box and the cylinder head. But most Evos suffer from the type of leak I first described. Hope this helps.
[edit to add] About Evos and gaskets--you may see some seepage at your cylinder bases. The original HD base gaskets were paper--not good. If they're not leaking yet, you can stall for time by being judicious about warming up the motor before taking off. Let it fast idle, about 1000-1500RPM from cold until the rear rocker box is too hot to touch. Not roasting hot, understand, just at the point where you can't comfortably put your bare fingers on it. This will allow your cylinders to warm up gradually and go easier on the gaskets. Regardless, it probably won't be too much longer before you're going to want to replace the base gaskets, if they haven't already been replaced. Use Cometic or James brand base gaskets and still use the warm-up procedure I described.