Sounds like a head gasket... the starting resistance is likely liquid in the cylinder(s), which may be leaking past rings into the crankcase - hence the milky oil.
It needs proper attention ASAP.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"