With aircraft under a maintenance control system eg RPT and charter it is not unusual to get CASA approval to over run some checks by a few hours. In commercial operations I have certainly seen extensions approved up to 10%. I don't know the legal answer to your situation as a private operator but would strongly advise that you get some sort of approval in writing to cover yourself in case of an incident. In practical terms, if the engine has had regular use then running it 5 hours over the oil/filter interval is highly unlikely to cause it any harm. Maintenance intervals for aircraft are usually conservative.