Well general rule of thumb, if you go past 15 use the highest octane. We do have different scales, ours will be lower than what yours is. I just use midgrade cause I have always done it, figure I better treat this car better than my old one. People run 17 and more and run midgrade, but IMHO thats just taking the life of your engine in your own hands. Stock is 13-15, so going to the highside of 15 should be safe with low grade, possibly get away with 16 on low grade but i would go to midgrade myself. Past stock timing you run the risk of detonation in the 4500-5500 rpm range I think. Something like that and the upper end could do it....probaly just talking out of my ass tho.