I wonder why you would want to bother with synthetic oil? I believe it costs more but in a modern car it will last longer between changes. With a Model A, unless you have a full flow oil filter where ALL of thye oil from the pump gets filtered, you have to change oil on a regular bases to get rid of the contaminates. This would negate the advantage of the extended change interval of the synthetic. As for as lubrication, a good quality oil will do just fine in a Model A.
Again, this is just my opinion.
Chris