Amsoil is a great oil for diesel's and should be for gassers as well. A friend of mine has is just over 20k on his last oil change.

Now the best way to know how your engine/oil is doing is to get a sample every 3-5k and send it to Blackstone Labs. Blackstone
They can tell you how it looks and what it contains. It's pretty cheap as well.

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2001 F250 Larait 6spd
1994 Ford Lightning
1997 Rodeo