Oil Recommendation for Early Engines
For early engines, please determine what bearings with which your engine is equipped before trying new oils. It is very possible that the additives of ANY modern (as in not non-detergent SA) engine oil would have a negative impact on non-tin/lead (such as silver-lead) babbitt bearings. If the engine has tin/lead babbitt or shell insert bearings, any modern oil would likely be suitable. Shell insert bearings were introduced around 1933. For early babbitt bearing engines, castor oil may still be the best lubricant.
Another concern with older engines is PAO oil's effect on internal components made from Polyoxymethylene plastic (aka Dupont Delrin). PAO oils can potentially cause stress cracking of plastic engine parts made from this plastic. A Group III oil may be a better choice in engines having internal Delrin plastic parts.
Many Model T owners have upgraded the friction linings in their transmission bands from cotton to Kevlar. The use of Kevlar completely overcomes the problem of cotton fibre disintegration but creates a transmission slippage problem instead. To reduce Model T transmission slippage with a Kevlar upgrade, I don't recommend the use of Starburst oils as these contain friction modifiers that can exacerbate transmission slippage. Motorcycle or HDEO oils suitable for wet-clutch applications may be a better oil in Model T applications. Another oil potentially suitable for Model T transmissions with a Kevlar upgrade is an HDEO similar to Mobil Special 30 which would provide better sludge and wear protection than a non-detergent oil like LUBRIPLATE Non-Detergent Motor oil. In colder weather, a Type F automatic transmission fluid may provide better slip protection but does not offer any detergency with a resulting risk of sludge. The viscosity of a Type F ATF would be similar to a 20-weight oil (see Mobil Type F ATF as an example).