I know this has been discussed to death on the forums here but I
just got off the phone with a Castrol engineer who said they no longer recommend ANY SM-rated dino product in a flat-tappet engine. As they continue to reformulate to appease the auto industry, (read, extend catalytic converter life) the zync content continues to decrease. The only thing they recommend for older engines now is their synthetics.
A couple of years ago the GTX series Castrol products still contained quite a lot of zync compaired to other brands and I've been a Castrol user for years but I'm getting a little worried about this now. I'm thinking It's time to switch to Shell Rotella or Mobile Delo. (A recent Hot Rod magazine article also recommends switching to diesel oils for flat-tappet engines).
