No don't use it unless it was always used , instead add a litre of Lucas oil treatment instead of one of the litres of oil when you change your oil next.
Hope that helps and best of luck.By the way why you don't use synthetics unless it's been used from the start is it could damage your seals etc inside he engine and Lucas helps to coat the engine , helps just in case you over heat or go low on oil and it also may help to give the engine a bit more power.