Synthetic oil + gas mileage

I have several questions regarding synthtic oil. Have others noticed a gas mileage increase with synthetic or synthetic blend oil?
I just switched to synthetic blend in my 1996 Integra LS about 6000 miles
ago. I noticed my highway mileage went up about 2 mpg, from 32 to 34 with the switch to synthetic. Is this similar to what others have seen?
I was having the oil changed at 3500 mile intervals, but went to 5000 with the synthetic. I have 80k miles, drive about 8000 miles a year, mostly highway. Is a 5000 mile interval reasonable, or could I stretch it to 7500? 7500 would end up making it an annual change. I live in central North Carolina, where it hits the 90's in summer and the 30's in winter.

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