Hi JackandJanet, I have gone up this same grade for 25 years and it is not so extreme that I have ever needed to lock out OD, have had 5 5.4's since 97 with the same 3.73 and 2 with the 3 valve so I have a pretty good idea how these trucks run. Surge is only in the 87 tow, like I said I will try the 91 perf as soon as I suck about 35 gallons out of my 36 gallon tank.
John.