[quote author=maudi3 link=topic=66503.msg1143073#msg1143073 date=1168467762]You guys do know that BITOG is an Amsoil Distributor's site, right?[/quote]
It was started by an Amsoil distributor, but he doesn't own or operate it anymore.
I use synthetics in winter and conventional oil in summer. In my 3, I've been using Tech2000 or Motomaster 5W-20 in summer and Mobil1 0W-20 in winter. Synthetic oil, especially the Group III stuff, will not necessarily cause leaks and there's no problem going back and forth. I believe that some synthetic base stocks in the Group V category (esp. esters, from what I gather) can contribute to seal leaks in older engines due to natural detergency, provided there is some build-up there to remove. Group V base stocks aren't used much due to cost anyway though. There would be minimal amounts, if any, in anything you can buy at WalMart. I don't disagree with you, maudi3, on the issue of smaller and more consistent molecules in synthetics being more likely to leak out seals if the clearances are already large enough for that.
My recommendation is to ignore the buzzword "synthetic" and just use an oil that meets the specs you need, is suitable for your temperatures, and has a name you trust. Don't fear an oil because it is synthetic, but don't assume it's anything special or even superior to the dino oils on the shelf either. Follow the manual and stay with an SAE 20 oil, usually 5W-20, to avoid potential warranty hassles. Heavier oils will work fine, and possibly even protect better by an insignificant amount, but will increase fuel consumption. API SM/GF-4 is required, and it's nice to have the Ford WSS spec on there too. That spec just means it passes some additional wear testing. If you go below 0F, I recommend the oil be 0W rated too.