I emailed amsoil once about that. their reply was that on very old, non rebuilt engines, ther emight be some materiall incompatibilities.. could have been a CYA answer.
I used used it in my 63 ford 4000 after a bout of stuck valves and bent push rods.
then again.. I went back in with new gaskets and such, and some of it looked like it had been rebuilt in a recent decade anyway.. so I took my chances.
the 89 cat will be no problem for sure.. just use a C rated oil.. etc..
soundguy