an area normally outside of our expertise and jurisdiction
Of course it is, we see proof regularly. See how you get on if caught running your non STC'ed engine on mogas. Does CASA actually know anything? Can I fly my lighty if the temp exceeds 40°C yet? CAO 20.9
3.1 The pilot in command of an aircraft shall ensure that the aircraft is not flown unless the aviation fuel, aircraft engine lubricating oil, aircraft engine power augmentation fluid and aircraft hydraulic system fluid used in connection with the servicing or operation of the aircraft complies with the specification and grade required or approved for the purpose by CASA.
Note 1 In respect of aircraft engine power augmentation fluid and aircraft hydraulic system fluid the specification and grade specified for a particular purpose in a manual or manuals promulgated by the aircraft or aircraft engine manufacturer may be considered as having been approved by CASA.
Note 2 The pilot in command may assume that:
(a) aviation fuel; and
(b) aircraft engine lubricating oil; and
(c) aircraft engine power augmentation fluid; and
(d) aircraft hydraulic system fluid in the aircraft, other than that which he has caused to be delivered into the aircraft, complies with the required specification and grade.