Can you actually identify where the cowl edges are butting up against the fuselage skins? Is there paint removed from the metal?
In my experience the black streaks after flying in IMC or rain are more likely to be hinge pin lube or aluminium dust - or general engine minor oil leaks/dirt/grime making their way out.
So worth checking for that just in case.
RV7 VH-XRC Sold :-(