Engine:
Capacity is simple, you just top up the sump.
The owner’s manual for the B29 state they were using Wakefield Castrol XL, Mobile TT, or Shell Triple. Other handbooks for the Douglas big twins of the same era mention in addition Wakefield Castrol XXL summer use. This equates to 30W and 40W respectively. You can run it on something lighter, or a multi-grade, in a pinch. It will not harm anything other than smoking a bit more and sooting up the plugs and combustion chamber.
Any engine oil you get today, even the mono-grades, is likely to be detergent oil. This could be a concern as the B29, unlike other Douglases up to that point, is NOT a total loss system, but re-circluatory. Were it total loss, the engine would only ever have seen clean oil, but in a re-circulatory system you can get sludge build up. There may be sludge deposits in the engine that could be loosened and distributed about by the detergent in the modern oil. Do the first oil change after running the engine an hour.
A cup of oil poured into the timing chest would be a good idea, till the oil pump has a chance to circulate enough oil to build up the level in the timing chest on its own.
Gearbox:
The makers recommended Wakefeild's Casroleum or British Oil and Turpentine Co.'s Crimsangere, or Shell grease.
I use standard wheel bearing grease, thinned a slight bit with 90W gear oil to make 'groil'. At 70F deg it should self-level in about five minutes after dragging your finger through it. Straight grease will stay in the box better, but the sleeve gear lubrication is a bit marginal so I would rather have something a little thinner that has a better chance of getting in there and doing some good.
As for how much, that is a very good question. Douglas never did state a capacity in any of their prewar handbooks. They just say it is best to add a little every 400-500 miles. On straight grease you pretty near pack the box, and the gears cut trough through it. On 'groil' you should only need to submerge the lay shaft. You will not hurt it by over filling, and the excess will quickly find its way out! It sort of seeks its own level. If it is not leaking out the sleeve gear, you probably need to top it up! If I recall correctly the B29 type box should have a filler plug/fitting on the front side of the box. Take this out, and fill to the level of the hole.
-Doug