Fx4 5.4.. 3.73.. Here in parts of Colorado there Is 85 octane..Is It ok to use In the 5.4..?? or will It create problems later..??

 

Ford "does not recommend the use of gasolines labeled regular that are sold with octane ratings of 86 or lower in high altitude areas". Other than that I could not tell you what it may or may not do for your engine.

 

Typically, you want a lower octane in higher altitudes. Because of the lower percentage of oxygen, the lower octane creates a faster burn. At lower altitudes, this fast burn is detrimental but not at higher altitudes. Your trucks computer should detect a ping if it occurs and retard the spark.

Amazing how much money is wasted on high octane fuel. People have it in their mind that their is more energy content in a high octane fuel vs low octane fuel.

The only reason to use high octane fuel is if you have pre-ignition or if the fuel mileage increase due to using higher octane fuel is cost justified vs the higher cost. I have a Lexus GS300 that specifies premium fuel but only gets around 5% better fuel economy with hi octane. It is more cost effective to use regular.