I read somewhere that avgas can be used to flush the crankcase after the old oil is drained and prior to filling the crankcase with fresh oil.  A gallon of avgas is allowed to sit for a few minutes in the crankcase in order to thin the residual oil and to keep it from contaminating the fresh oil.  What is the general consensus about this practice?  Are there any good reasons not to flush with avgas?  Is there a better alternative fluid?