My gsr has a cammed b20. I was driving it on the freeway around 4000 rpm and I was wondering if thats too high of a cruising speed. I do not beat on my engine. Latley it hasn't been sounding quite as smooth. I hear a slight vibration and little clacking sound when I listen to the engine. A friend of mine said that its just my valves and it could be from me using the lower octane gas. Could this lead to a problem? I ran Sea Foam threw the intake manifold 3 times and the engine still sounds the same.