The right viscosity grade depends on engine design, engine temperatures, and climate. If the air cooled engines run hotter, then a higher viscosity and a more narrow range may benefit, but engine design and condition are also factors so it is hard to say. And you may need a wider range to allow cold temperature starting.
In general, modern tribology brings better additives and base oils than old formulations and should be suitable for older engines along with the right viscosity grade. If I could not find any oil recommendations from my engine manufacture, then I would lean toward a thicker oil in old engines, such as 10W-40, or 5W-40 if cold starting is a factor. I am not a mechanical engineer so this is just an opinion. I don't know what factors affect oil filters.