I wouldn't waste my time with the
synthetic blend. If you want to do that, mix it yourself using a 50/50 ratio. If you look on the bottles, nowhere does it tell you what the syn/conventional ratio is. I contacted Castrol a couple years ago and they said it "could be as low as 20%
synthetic". The price per quart sure didn't reflect that ratio.
I usually change
oil every 5k. There was an in-depth study done using Mobil 1 in a late model Camaro. They went a whopping 15k miles before the
oil broke down to the point of not being usable. Mind you though, they did add 1/2 quart here and there.
Most automakers say not to start using
synthetic till you have about 10k on the engine. If the rings aren't seated properly for some reason or you have some other problem, you'll see problems using synthetic due to it being so slippery.