lowest grade fuel is what everyone uses for their family car !!!!!
it will not do any harm to your engine
high grade fuel to just to increase engine performance
so if your car is not a sport car ,don't worry about anything
although using "lowest grade fuel" is not as good as compare to using "highest grade fuel" but you are saving $3-$5 every time you fill up your tank,and it will not do anything better to your car, i would rather get good oil change more frequently than using high grade fuel
good frequent engine oil change should be what you need to think of,not fuel !!!
i own a 2004 toyota carmy, had 76,000km or 47,000miles
my engine is kind of the same as yours
all i ever used was lowest grade fuel, have not even spend a cent to reapir the engine, all i used was lowest grade fuel from ESSO gas station
remerber!!! fresh,clear engine oil is what keep your engine run longer!!!!!!!!not fuel