i have pontiac firebird 92 v6 3.1 112500 miles on it. i have a small oil leak,probaly rear main seal. it is leaking somewhere back of the motor and a bit high,about a bit higher where oil pan is.how big or expensive thing it would be to change rear main seal? is there need to take engine or transmission apart? one car mechanic told me that it takes maybe six hours to fix the leak. what you guys think??