I run 9' oars on my sports car (12' cat) at low water or narrow ie. Clear Creek, Boulder Creek, low water Numbers, etc.
I run 10' oars on my sports car at high water, wider rivers or on my gear hauler (16' cat) for short trips.
I run 11' oars on my Grand Canyon rig.
With 3 different lengths, the extensions come in real handy so my spare always matches what I am rowing.
I brought 2 extensions on the last Grand trip, thinking they would be nice added to the 11's for a windy day or the row out on the lake.