95 RON tuned car with 91 ~ less efficiency, increased engine wear and less power, not worth it
91 RON tuned car with 95 ~ a little bit more efficiency/power maybe, not worth it
91 RON tuned car with 91 ~ maximum efficiency
Airfilters and oil are the two primary maintenance requirements in a modern car. Another one which most people forget is the the inflation of tyres. They are reasonsably cheap, and if you have a popular car just wait for Supercheap or repco to have a sale and get a discount on oil and filter, stock them up if that is your kind of thing. I generally went up a grade in oil as well whenever I can, just becareful how quickly you change grades as cars get used to certain grades, but I am running our car on fully synthetic.
Other maintenance can generally be scheduled by observing changes in the way the car drives. Noises when starting stopping and turning (low speed especially) are the ones to look out for. Plus VTNZ are pretty good at pointing out things under the car which you don't generally notice.
Last one to keep in mind is cam belt vs timing chain.