Please note that Packard originally recommended 20 W oil for our cars. Please see the picture here.
If the engine is in reasonably good shape, it would be best to stay as close to the original viscosity as possible.
When the engine is hot, the oil flows best to all the components. So many experts recommend to stay as close to the viscosity at full temperature as the original oil.
It is oil flow (movement) that gives the best lubrication.
See the graph here that shows viscosity comparisons of straight weight oil to multi-viscosity.
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