I know that BMW says to use 89 octane for fuel and i do all the time. But if i used 87 would it make all the much difference? Cuz where i live 89 is about $3.27 a gallon compared to $3.09 for 87. This 89 is startin to take a tole on my wallet and since im a teen i dont exactly have a high paying job. so would it make a big difference or no?