Diesel.
The gasoline that is recommended by the manufacturer is the best fuel to use.
No. If your car runs well on a lower octane gasoline (i.e. no engine pinging or knocking), then putting a higher octane gas in your tank is just wasting money...
yes and more horsepower
No it does not give you better gas mileage, all it does is helps with any loss of horse power and white smoke coming from the tailpipe due to burning antifreeze.
63km/ltr
No it will not give you better performance or increase gas mileage. Gas mileage and performance will decrease only if you use it. leaving it off is like not having it at all.
In terms of gasoline specifically, presumably premium gas would be best, although I think diesel is supposed to give you the best gas mileage.
It could, if the old gasket had failed.
Yes it can, depending on your driving habits.
No it does not. But it does give you somewhat better gas mileage.
Bicycles give you more gas mileage.