There is a crush washer under the plug. It should be
replaced with each change. It is made of aluminum and
costs less then a buck. Some other models of Volvo use
copper crush washers -- copper should not be used on the
800 series.

First thing to determine is if the crush washer is being
replaced and is it the aluminum type. Also might want to
make sure it is from the oil pan bolt since leaks higher up
can run down. Do you have a model with a turbo? The turbo has
an oil drain line that often leaks (it is an inexpensive repair).
GLT models have turbos starting in 1997.