FWIW, it gets hot in our garage too. My unscientific 2 cents is that I think grease and oil dry out faster in the heat. Anything rubber seems to suffer some (plus the leather saddle/toestraps dry out), but I think it's ozone (smog) that's actually harder on the rubber than the heat. I know for sure non-bicycle stuff (snowshoe bindings) made of neoprene definitely falls apart in the heat over time, so on a bike that could include various straps. Let the air out of the tires, or leave just barely enough to hold shape. I think that stuff stored near our gas hot water heater and furnace might conceivably oxidize faster???
Reid Fisher getting warmer by the day in San Martin, California