Don't know about amsoil but I run 75/90 Mobil-1 and at 180,000mi it has been good to me. I don't know if the ARB's make any difference in the grade oil, you might want to look for a 140W.
WHat you're probably using is the synth mix or whatever they call that as valvoline doesn't put out a full synth at that weight (That I could find anywhere!)