We have an 0-320 in our Grumman.  When I was looking the plane over for
the first time prior to purchase I noticed only about 5 1/2 quarts on the
dipstick.  The previous owner told me he didn't bother putting more than 6
quarts of oil in because it just "blows out the crankcase vent."

I tried an experiment and found that the engine "burns" about 1 qt/hr
until it gets down to 6 in the sump; about 1 qt every 7 -9 hrs from there
on.  It seems each engine is a little different.

Mark & Lisa Sletten
Legacy FG N828LM
http://www.legacyfgbuilder.com