Don I think engine oil in general, as well as engine emissions, have come a long way since the '70s. Anyone who didn't change oil religiously back then accumulated engine deposits much faster then than now. I have seen inside a lot of motors from the '70s and '80s and most looked nasty by 100-150k. Take one of those older engines even ones with less than 50k miles and I can see adding Amsoil would be a disaster. I have a '68 Cougar with 130k on the original untouched engine. There is no way in heck I'd put a drop of Amsoil in that motor, I'm sure it would ruin it in no time. But someday if I rebuild it, I'll Amsoil it right after break-in.
Today's cleaner burning engines with higher-detergent dino oils can get away with higher-mile switchovers than back then but it still should be done with caution. The good thing about our CTDs is they don't seem to build up sludge and deposits very fast, and it's mostly soot at that, which doesn't have the clogging cruddy consistency gas engine deposits do. For those who start off early on Amsoil and use it properly and sample it on long drain cycles I don't think they can go wrong.
I don't think Amsoil or synthetic is the answer for everyone. I mainly like it so I don't have to change oil as often, and I think it provides an extra measure of protection especially when I'm towing in 100+ temps or starting on a cold winter morning without being plugged in.
Vaughn
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