When we burned 115/145 in our 150s, 172s, 177s, and 310s back in the mid-70s, we had to lean the engines ALL the time to prevent excessive spark plug fouling -- not just above 5000'. IIRC, the 177RG was the most problematic because the CHT would get very close to the red line in a climb on hot days. It was a close balance between fouling and overtemp...