Reason for me mentioning this is that I'm experiencing pretty much the same problem as you have.
Once the engine warms up (180-190 deg) it starts pinging when leaning on the throttle.
I've mentioned it earlier in one of your topics but it never got picked up.
I went from a stockish hydr.cam with 1.6 rockers, to a solid XR274 roller cam w/ 1.5rockers and the engine started pinging when pushed (Besides/because the higher cylinderpressure ofcourse).
In my case I'm pretty sure it's the underhood heat and hot air that is causing the engine to ping, compared to the fuel-octane.
For testing winter/summer blends, you could fuel up a few canisters with summer-fuel and save them for test use in wintertime.
