Hey all,
AMSOIL has been making synthetis for years. They have had an oil on the market now for about a year called Z-Rod. It comes in 10W-30 & 20W-50 grades. AMSOIL Z-ROD™ Synthetic Motor Oil is specially engineered for classic and high-performance vehicles. It features a high-zinc formulation to prevent wear on flat-tappet camshafts and other critical engine components, along with a proprietary blend of rust and corrosion inhibitors for added protection during long-term storage. Z-ROD Synthetic Motor Oil is designed to perform on the street and protect during storage.
Here is the information and pricing for this oil:
http://www.amsoil.com/storefront/zrt.aspx
1970 Duster
The main reason for the high levels of ZDDP is to protect flat-tappet cams, lifters, rockers and other areas susceptible to wear. Its high-zinc, high-phosphorus formulation provides the extra wear protection these critical splash-lubricated components require. Your roller style valve assy dosen't produce as much heat and friction as the old style does hence the reason for the ZDDP.