Need Help with my DOHC ZC

OK i drive 37 miles to work daily, so every 2 days (monday,wednesday and friday) i would put in $10 of 93"ultra" from sunoco. I would get alittle more then half a tank. But with the weather getting warmer now, i dont get as much out of my gas, do u think its cuz the air is warmer and the ecu has to adjust to the ambient temp. or could a dirty O2 sensor be giving bad readings. Also with an I/H/E/MSD should i run cooler plugs then the stock NGK for a 89teg LS? If so or not what would be the best setup for this motor? And finally.... I burn oil so i put in 10w40 Mobil, i tried using 20w50 Mobil oil but felt that it reduced my engine power alittle, so what should i do short of rebuilding. :banghead: