Without a doubt overdrive transmissions is the biggest factor.
EFI is better yes, but even they have to get run out on the highway to "blow the carbon out" every now and then.
I don't know about the significance of overdrive. Lots of new 4cyl cars still buzz at 3000rpm down the highway and they have to still meet the same standards of longevity as lower revving engines.
Also, if you've ever worked on an engine with carbon build up, you'll see why a full throttle run on the highway isn't blowing anything out. Don't know where that myth came from....
