I just bought a 2006 Explorer xls with 32k miles on it. It has the usual 4.0L V-6 SOHC engine. I bought it from a Ford dealer so I'm assuming it came with the Ford recommended oil, 5-30 Motorcraft Syn. blend (I will ask them.) I have no idea who owned it before me although Car Fax claimed it was a one owner vehicle. My last 2 Explorers I bought new and ran Castrol Syntec until they had a lot of miles, and then switched to high mileage synthetics. I can't complain about the results....I had 165k+ on both vehicles, but had a front seal leak on the last one. A dealer told a friend of mine you can switch to synthetic from conventional, but don't switch back to conventional oil from synthetic. I'm not sure I believe that. Some knowledgable folks claim synthetics dry out seals. I'm probably complicating this, and most oils would probably be ok as long as it's changed reasonably often. Any feedback is much appreciated. Thanks!