Anytime Smokey says something's good ... it's got my vote.

I really wish they still made Arco Graphite ... the Black Oil. That stuff was AWESOME!

 

I am having an exactly similar issue. I have been running Amsoil 10/30 in my 97 360 magnum since 51,000, now I am at 86k. I have changed regularly. I actually still change every 6 months because I over maintain sometimes, its cool, anyhow, this time I stepped down to 5/20 to try to reduce some parasitic drag in the motor from the oil weight. I am now getting some light lifter knock, I assume, under load. At idle not anything at all. I am planning on another oil change before the regular interval here and going back up in weight as I noticed no power increase from it and the only differance I noticed was the lifter knock. Try a thicker oil.