The distinction between oils that have two ratings, in this case 10w30 versus 30W is that the former oil has a wide viscosity curve for use in colder weather. When the temperature is low, 10W30 has the ability to act like a 10 weight oil, so that the engine still gets lubricated. In warmer weather, it acts like a 30W oil. There is no effective lubrication difference between the two, except for the vicosity curve. Amsoil make an excellent high zinc content 10W30 synthetic that is intended for use in vintage engines. You can find it on their website. Here is their blurb on this oil. An excellent product.
FROM THE AMSOIL WEBSITE
Protects Flat-Tappet Cams
The lifters and cam lobes on flat-tappet camshafts common to classic and high performance vehicles slide rapidly against one another, producing high friction and heat. The friction between the two components can eventually wear down the cam and affect valve operation, ultimately resulting in lost engine power and reduced efficiency. In addition, these areas are splash-lubricated rather than pressure lubricated like other areas of the engine, which adds extra strain on anti-wear additives like the zinc and phosphorus in zinc dialkyldithiophosphate (ZDDP).
AMSOIL Z-ROD Synthetic Motor Oil is formulated with high levels of ZDDP 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.
Provides Long-Term Protection from Rust & Corrosion
AMSOIL Z-ROD Synthetic Motor Oil is formulated with a unique blend of rust and corrosion inhibitors to ensure maximum protection during long-term storage. To prove its effectiveness, AMSOIL submitted Z-ROD Synthetic Motor Oil to the Standard Test Method for Rust Protection in the Humidity Cabinet (ASTM D-1748). This test evaluates the rust-preventative properties of oil under high-humidity conditions, similar to those faced by a covered hot rod in a damp garage. The metal coupon treated with AMSOIL Z-ROD Synthetic Motor Oil showed no signs of oxidation after 192 hours, while the coupon treated with a leading competitor’s product failed the test after 24 hours.
AMSOIL Z-ROD Synthetic Motor Oil is recommended for older or modified engines requiring either 10W-30 or 20W-50 motor oil. Z-ROD Synthetic Motor Oil meets API SL and earlier specifications, allowing for increased levels of anti-wear additives. ZDDP levels in Z-ROD Synthetic Motor Oil exceed the limits of API SM and newer specifications.
AMSOIL Z-ROD Synthetic Motor Oils are compatible with conventional and other synthetic motor oils. Mixing AMSOIL Z-ROD Synthetic Motor Oils with other oils, however, will shorten the oil’s life expectancy and reduce the performance benefits. Aftermarket oil additives are not recommended for use with AMSOIL Z-ROD Synthetic Motor Oils.
Because engines in classic cars, hot rods and other performance vehicles are generally modified, a universal oil drain interval recommendation for these applications cannot be given. Responsibility for determining the drain interval duration rests with the owner. As a general service guideline, the maximum drain interval for Z-ROD Synthetic Motor Oil should not exceed 5,000 miles or one year, whichever comes first. In heavily modified engines (e.g. forced induction, nitrous) the maximum drain interval should not exceed 3,000 miles or one year, whichever comes first.