Updated: 25 July 2020 – Still no issues with the motor. Also fixed up some errors and typos in this post.

From the “Individual experience not matching conventional wisdom” files…

The NZ AA have expressed concerns surrounding the “D4” (including the 1AZ-FSE) or GDI (Gasoline Direct Injection) motors on their publications due to issues of Potential carbon build up on the intake valves, potentially restricting the airflow to the engine as a result of the petrol being injected directly into the combustion chamber as opposed to washing over the intake valves as per normal port injected engines.  They also recommend running Toyota D4 motors  on the “Highest Octane possible”.

In my own experience…

Having acquired my Toyota Avensis (With the D4 1AZ-FSE gasoline direct injection engine) at 100,000km, I have driven it to 225,000km and apart from an Oxygen sensor failure, I haven’t noticed any problems with this engine over and above a normal car of similar age and mileage.

According to my own record keeping at each filling station, running the Avensis on 98 RON fuel (from BP, Mobil) has not yielded fuel economy nor performance benefits over and above 95 RON fuel (from BP, Mobil, Caltex, Z Energy).

• For 9 consecutive fills of Caltex Techron 95 RON… Fuel economy: 9.4 litres per km (Mixed Urban / Highway). Range 9.0 to 9.6 litres per 100 km. Date prior to 15 April 2017
• For 4 consecutive fills of Mobile Supreme+ 98 RON… Fuel Economy: 9.8 litres per km (Mixed Urban / Highway). Range 9.5 to 10.00 litres per 100 km. Date after 15 April 2017

In regards to performance. There has been no discernible increase in performance, in fact, there may have actually been a loss. In the scenario of driving up the southern side of the Bombay hills northbound on State Highway 1 (with myself, no other passengers and one bag 20kg of luggage) which I regularly undertake…

• With a tank full of 95 RON Caltex Techron Fuel, the car is able to maintain 90-100km/h in top gear (Automatic transmisson – 4th Gear)
• With a tank full of 98 RON Mobil Supreme+ Fuel, the car was constantly hunting between 3rd and 4th gear

During an earlier (in 2013, when I first got the car) consecutive run with a few tanks of 98 RON Fuel, there was actually a mileage drop of about 5-10% using 98 RON fuel as opposed to 95 RON fuel. Later tests (in 2017) indicated little difference in economy, though still tending towards lower economy.  In the earlier 2013 tests, it appeared much of 98 RON fuel range included 10% ethanol which has less energy content than straight petroleum and may explain the drop in Fuel Economy. It appears Mobil and BP have since removed ethanol out of their 98 RON Fuel Range.

The Higher detergent ratio in the 98 RON fuel range with slightly less base gasoline per litre (Potentially slightly less energy) may be affecting fuel economy, along with the possibility that the 1AZ-FSE motor is not tuned, configured or otherwise designed in a way to take advantage of the higher anti knock rating of 98 fuels.

In short, YMMV quite literally I guess. I am in no doubt that for cars that are designed / tuned to it / are able to take advantage of it (such as extra high compression motors, and Turbo Charged vehicles), 98 RON Gasoline will be beneficial in performance and /or economy, but individually in my naturally aspirated D4 Avensis specifically, not so much, in fact, 98 RON in my car specifically has been a waste of money. (Higher price per litre and no gains in mileage nor performance)

See also…

• Widening price gap between 91 and 95 at Caltex