Run it my car last 1000 miles before oil change and in my fuel - 2001 C320 Benz - runs much better with seafoam in gas - as a flush in crank case.
Change oil so often in boat don't need it crank case. I add Hy-Per Lube made by these guys listed in below link for extra wear protection on my flat tappet engine...
+ VR1 Synthetic 20w50
And yes more fluid will displace oil yes 5+ but the seafoam will reduce viscosity a bit. Run it in my outboards they run very well season to season with it.