Should I install an air conditioner pad on existing unit?
Most units require a pad when installed . at the age of the unit I would probably not try to put a pad underneath as this may create a leak. In 99 the common refrigerant was R22 and the cost of a recharge if a leak is created may cost close to $1k as r22 is being phased out and local shops in my area charge 100 per pound.gravel around the unit could keep grass or weeds from growing close to the condenser coil but at this age I would not want to move it but want to keep it clean so it will last as long as possible.