Fins?  YOU GOT FINS on your WBX crankcase?  Not likely that the specific
heat of the two oils varies much but the mechanic was pretty much spot on
with his advice for aircooleds.  They're really mostly oil cooled, with a
large amount of heat being transfered via the oil to the case and sump and
thence to the air.  If the oils have significantly different thermal
capacity characteristics then ... well you get the idea.

p