engine oil

Can somebody please tell me what the correct type of oil is for my 1939 A?

There will be many posts to follow, but My experienced opinion is to use 10-40 oil that you would use in a car. Good quality name brand oil is best. Others will say use single wt oil like 30.
or Non-detergent oil. Do not. The oils of today are great, and are making engines run for far longer than any oil of yesterday. If you do not know the history of oilin the engine, after changing the oil & filter, run it for an hour to temp, and look at a sample. if it is very black, I would change it, and filter again. Then change them after it shows very black again. THis might seem extreme, but that will help clean out residual trash and deposits. If it stays pretty clean, leave it in. Change it yearly if it is light use, more often if not allowed to warm up in use, and you should be fine.
There are viscosity index modifiers in multi wt oils that can be unstable if extremely overheated. (as in the engine was at 350 degrees till it seized) Your engine will never see those temps, so the issue is moot. The lower number is a cold condition thinness number allowing the oil to get to and lubricate things when cold, as well as to allow the engine to rotate and start when cold.
The upper number is the viscosity number when the engine is at temp, and it indicates that the oil never gets thinner than a 40 (in this case) wt oil would get when at temp. This thicker oil cushions and protects better than a thinner oil, and maintains better oil control in the cylinders.
There are specific numbers that go along with this data, and are available on line from many sources. JimN

10W 30 in a cool climate, 15W 40 anywhere, SAE 30 if you live where the temperature is consistently above about 40 degrees, all detergent.

Use clean oil, and change as appropriate. there are many opinions out there on what oil to use. Multi-vis is fine. Straight weight is fine. Neither are fine if you don't change oil and filter appropriately.

myself, I run straight weight. 10 in the winter and 30 in the summer. The hours I put on the tractors about match up with when it is time to change the oil weight, so no worries there.

However, if straight weight becomes no longer available, I have no issues using multi-vis.

also, many people will debate the pro's and con's of each brand. Once again, regular oil changes when appropriate FAR outweighs the differences in any name brand oil. If you think that your oil brand needs to be on the side of an Indy car, then go for it.

as always, IMHO: In My Humble Opinion

Use a good quality 10w30 if a show tractor.
Use 30wt if you are working it in warm weather.

Do Not Use 10w40 ! ... 15-40 is Okay.

What is wrong with 10w40? My 4 wheeler recomends it. I have always used it in my tractors and pickup also with good results.

10W-40 got a bad name in the 80's, I think. Something about too much difference in the spread between 10 and 40. I don't know if that is true. My tractors use 15W-40, SUV gets 10W-30, Pickup 5W-30 because that's what the mfg recommends. Lot of rumors about oil, best to ignore them and use what common sense tells you.

I use Harvest King All Season Engine oil, I think it is 10w40 or 10-40 and have had no problems. My tractors all have very good oil pressure and don't burn oil with this oil. I buy it from L&M Fleet Supply or Fleet Farm.