S: Try contacting the Dealer . The Head should be removable allowing handle disconnecting point for the shaft housing removal to allow exposure to the Flex Cable. The Manufacturer implies a life time cable shaft warranty.
I have only greased my cables on numerous Older Trimmers every 2-3 yrs. I use a hypo id /moly grease combination . Like I said lubrication should be seldom . If you shaft indicates heat close to the head inspection is req'd asap . On Commercial Grade trimmers / brushers I have added a zerk fitting to enable a few squirts & greasing ease annually. These units however are used much more severely.