What grade of gas to use.
 

Have a 99 Ford F-150 ,has 147000 miles on it. Service engine light on . Have reset but comes back on. Should run a higher gas grade in it. Manual says to not run premium in it. Try to run non ethanol in it as much as possible. Please help

what makes you think the fault is related to the fuel octane rating?

if the mil is on. get the codes read and post it here.

Most engines today don't require premium fuel, although it doesn't hurt to use some once in a while. Regular quality maintenance is required, might consider doing as primem suggested and posting the codes here . Places like Autozone will get the codes out for you ,usually free of charge. Keep in mind that any codes obtained will not usually pinpoint the exact problem only the system with the problem and provide some direction . We can go from there.