Fuel Octane and engine reliability
As "mini-maxer" otherwise know as person with OCD I always try to maximize what I get out of fuel to the point that I adjust stroke and bore just below the point of knocking. The thing is it's not very fun or interesting as gameplay mechanic and it's yet another complication for new players. Another thing I noticed while I was recreating real life engines is that chances are that after matching performance of real engine you'll be left off with a lot of unused octanes. I bet a lot of people instead of designing their own cars will try to recreate cars of their favourite company and thing is as it is they would be pretty much penalized for doing so, not much problem now outside of forum challenges but we have to remember tycoon part of the game is coming.
So my suggestion is very simple, since reliability is abstract figure anyway why not add some engine reliability for left-over, unused RON? Logic behind it is simple, engine is simply more resistant to fuel that may be advertised as 95 RON but isn't quite there since gas station owner enriched it a bit with garden hose

Not only it would help beginners who probably aren't mini-maxing everything but it would also add some flexibility for more experienced players!