98 and good oil definitely helps - unfortunately most never check oil let alone use good oil

I was looking into that octane issue some months ago - carbon causing increase in compression, therefore the timing must be retarded to compensate (automatic in modern engines)

I am going to give https://www.repco.com.au/en/en/oils...trol-engine-conditioner-240ml/p/A9412003
and
https://www.repco.com.au/en/oils-fl...ystem-cleaner-375ml-adptsc375/p/A9672161

a go and see what happens (on a knackered b&s 35 just in case)