It is going to be in the 50's this weekend so my '51 Flat 8 is going to get an oil change. I need to know the capacity, no oil filter, and the oil weight. Additives aren't relevant, as I have stocked up on some ZDDP so I just need the proper weight. IIRC they were sticking straight 30 in them back in the day so I assume 10w30 would work but want to make sure.
I have used rotella 15w40 from day one. mainly because I run that in my daily driver which is also a gas burner and i want to keep the diffrent oils i stock to a minimum. I also run the rotella in my 4020 john dear, model D john deere, and 600 ford tractor. The flathead does not have real high spring pressure so zddp aditives are not that big of a deal as in an overhead valve engine with .400"+ lift. A flathead requires a lot less spring pressure than an OHV. However valvoline race oils still contain good amounts of zddp as they are intinded for off road applications. But any good quality oil will be fine. a dual viscosity oil was not available during the flatheads hey day but is a good choice because it flows better at lower temps than a straight weight oil.
i use 20w50 as my mains are a little worn so it gives me a little more oil pressure when its all warmed up. i also throw a quart of atf in there to clean everything out and keep the notorious flathead oil scum down to a minimum
i also use 20-50 in mine, (59ab), year round. engine is newly rebuilt. 38 lbs at idle-80 lbs when running. put in zddp for breakin, but not needed after that. ps- in 5 degree weather, instant pressure. in case your worried it's too heavy.