I am not usually an advocate of Sea Foam, as this engine well maintained and fed a steady diet of Mobil1 0-40 usually will not need it.
In your case, however, it appears your engine has been living off Havoline 10w30 or some other equally disgusting substance.
Give it the Sea Foam enema.

 

alaxfire is right: non-synthetic oils will more easily "boil off" thier lighter components and those will, in turn, condense on any cooler surface. Such as the cam (or valve here in the US) covers.

This is where "sludge" is born. Enough of this gunk will start to plug up small orifices and such.

Go cheap at your peril!