My dad sells motor oil for a living (Kendall mainly) and I have used a Ford F-150 Inline 6 300. I have used Kendall in my truck now for 5 years and after 112,000 miles it still runs like the day I bought it. I would have to say the Kendall is the best oil. Now I have been told that synthetic is basically a waste of money on gas engines unless you race around, basically run the engine hot or at high RPMs, the synthetic does cost quite a bit more, another benifit is that it stays more of a liquid in really cold weather and not quite as thin in really hot weather.

As far as a diesel oil goes, I'm not really sure but this link might be able to help you out...

http://www.kendallmotoroil.com/market/dieselenf.htm

Again it depends on what type of driving you do, if you offroad a lot it may be worth it to get the synthetic but for everyday driving I think your wasting your money.

Also when oils say that they go beyond 3000 miles, be careful, especially if your driving offroad with a lot of dust. The oil detergents and dispersants can only do so much and then the oil can't hold any more stuff in it.

Hope this helps, whatever you do don't run Penzoil, it's the worst oil there is!!!

One more thing, I hang out at the race track once in a while, if you get a chance ask some of the drivers if they have ever heard of Kendall, they will most likley say yes, and thats all I use. No kidding, even Nascar cars will use it, the same formula as you can get in the store.