Sunfish
Even under the standard MR/100hrly maintenance system there is nothing illegal about a privately owned aircraft running an engine trend log. The standard MR doesn’t provide for and really isn’t the right place for this information to be noted unless of course it’s outside limits. The important thing is the trend log isn’t being used to document items that are “Broken”. If though you have a dodgy auto pilot then it MUST be written up on the MR. Before further flight the auto pilot MUST be fixed or correctly labels as U/S and signed off accordingly by a suitably qualified LAME. That is the law and it is pretty black & white on this matter.