paw is giving you the correct information here.

It is not the weight that you are looking for as correct lubricant for this engine. It just so happens that 15W40 is what is termed Universal Grade and is what the truckers use. It has additives necessary for your engine that are not included in more standard engine oils, Castrol included.

The main additive that your engine needs is the zinc additive that has been dropped from most all oils except Universal Grade and most Racing Oils. The zinc additive is for health of flat and sliding cam followers. All modern engines use roller cam followers eliminating the need for this additive. The oil suppliers quietly deleted this from their additive package in order to put more profit in their pocket, but never bothered to tell anyone.

For your old six, use Chevron Delo 400 (my favorite,) Mobil Delvac or Shell Rotella T(has higher parafin content than the other two.) These are the most commonly available.

As an added bonus, these oils have more detergents that are used for soot dispersal in a diesel. These detergents will help prevent sludging in your older engines.