25% here and never had a problem. I've prematurely broke one con rod in 6 years.
Higher % nitro burns cooler as the combustion temperature of nitro is less than methanol, therefore the more nitro the cooler your engine runs.
You can use 30% and shim you engine, with little loss of longevity, however if you don't re-shim you engine it can kill it quicker. (check you individual engine instructions)
I use 25% cos I believe that shimming an engine reduces the power in order to get more power from the fuel, which seems a bit pointless to me. Plus nitro is more expensive and I can run lower % fuel if I can't get hold of my usual stuff without removing shims.
20% will be fine though. Most engines out of the box will run well on 16%-25% with no adjustments.