Once read an article written by a retired oil company executive in which he stated, in answer to this very question, that he would never use standard unleaded in his vehicles as the higher octane of 98 unleaded is a cleaner fuel and by virtue of that fact, will keep the fuel sytstem and engine cleaner. Apparently it will also keep the engine oil a lot cleaner between changes. Some engines are designed to run on higher octane. As for E10 at this stage I wouldn't touch the stuff despite Holden having endorsed it's use. Bit like cigarette smoking. many years ago no one gave a hoot and it was very socially acceptable to do so. Now, with the irefutable evidence available re the dangers to one's health from smoking (including 'passive' smoke), intelligent people won't go near the stuff.