概念，概念要清。。。Crankcase =\= 倒Engine Oil的那個口 :-)

crankcase is the housing for the crabjshaft.  to pour a liquid into it, you can 1) pour it from oil filler (就是你說的那個“倒Engine Oil的那個口”)； 2） inject from oil dipstick tube.
in your case, pouring seafoam into oil filler is fine.