Being newer to the hobby, I went through all the info I could find about oil/Gas/additives, including here on the Barn. When researching came to the same conclusions as the previous posters.
My opinions based on info I could find.
Research, choose what you think is best and are comfortable with. More important is changing when the oil starts getting dirty/black and change before storage. Your driving habits will determine how often you need to change. Should be at least yearly, if yearly change before storage.
Just about any detergent multi-weight detergent oil today is vastly superior to 1930s oil. Most motor wear occurs when the motor is cold, this is where multi-weight oils help. Detergents keep contaminants suspended so the contaminants are flushed during oil changes, instead of settling out of the oil and collecting in the bottom of the oil pan and other interior surfaces of the motor. Detergents do not loosen up deposits, as sometimes believed.
Additives are not needed for Model As due to the lower loads due to; weaker tappet/valve spring pressures used in the stock Model A motor, lower compression, and the lower rpms of the Model A motor. In later classic cars it is beneficial. Zinc generally was not in oil in the 30s, and was added later. There is info that too much zinc actually increases wear, so if used you should find out much zinc is in the brand/type of oil you use, and add only what is needed. Under 2000 PPM is OK. Additionally most oils of today have had Zinc engineered out, but other anti wear characteristics are engineered in. If you read the below link on independent testing, some oils without zinc outperform oils with zinc, so it is a crapshoot.
Some like to add a little Marvel Mystery Oil to the oil, for reasons I can not remember.
From my reading, current day oil is highly engineered, and additives can result in changing characteristics of the oil negatively. EXs; STP, Motor Honey, etc. - They basically thicken the oil, amongst other things. If adding a lot in 15w40, you may have changed it to 30w50. If this is your intention for a used worn out motor, OK, but perhaps using a 20W50 oil should be considered.
Here are some links on the subject, there are many more if you search here on the Barn.
https://540ratblog.wordpress.com/201...-test-ranking/
https://www.fordbarn.com/forum/showt...el+Mystery+oil