To set up an Oil Service Reminder without having issued any certificates, first you'll need to add the customer and job address if you haven't done this already:
Next you'll need to add a job address for this customer:
On this page:
Finally, in order to set an Oil Service Reminder, you will need to add the appliance for which the reminder will be set.
To do this, go to the page for the job address you just added (or the pre-existing one you wish to set a reminder for).
A window will pop up, in to which you can enter the appliance's details. Towards the bottom you will see these options: