I just changed from my winter mix to GC last weekend. It'll be my second summer with GC in my '98 Expy 5.4L. Last summer I noticed reduced gas mileage (about 5%), more sluggish engine response, and smoother and quieter engine operation. Previously I ran M1 5W-30 in the summer and M1 0W-30 in the winter.
Next winter I may mix 3 qts GC with 3 qts M1 0W-20.