Wow...unless you put very few miles on your car I'd question the wisdom of changing your oil only twice a year. If you read the synthetic bottles it says that you can go 7,000 miles between oil changes (instead of 3,000 on regular 10/30 type oil).
It's not just the savings in gas mileage to consider....synthetic oil is proven to be better for your transmission. I have 180,000 miles on my 2001 van (purchased in July 2002, so technically 5 1/2 years old). A lady at work has the same car, purchased in 2001, but just over 100,000 miles yet and she had her transmission replaced a couple months ago. She blames a "junk van", but I don't find the same experience with mine! Back when I bought my van and we were comparing notes, I told her I planned to use the good oil and she told me I was wasting my money. Hmmmm, her tranny replacement had to be what 1500 or more? And of course the cost of peace of mind knowing I've not had a recent catastrophic failure = priceless.