I've always used Castrol in my 98 and never had an issue in 206k. My Camel has always ran Amsoil since new and also, just fine with 176k on the clock. I have not torn either D1 apart to compare, but I do not think Amsoil will hurt it at all. Neither truck ticks, smokes or has had any bearing/trans or diff failures. In fact, my 98 has never had the heads off. Camel had heads done under warranty at 35k and been fine since. Amsoil is good stuff too.
2003 Disco SE7 expedition prepped
1998 Disco LSE 7 50th anniversary Baluga Black
1996 Disco XD Trek number 4 and Camel Selections
1969 Series 2A 109