15w40 will work fine. There are plenty of options though. You can run a 10w30 in colder climates. Some will suggest 5w40. Just make sure it's a diesel rated oil. ( C rated)
There are probably 1000 oil threads on TBN
Oil Capacity: not sure. Add three quarts and check the dipstick. Just add a little more until it's in the operating range. Start it up, wait for oil pressure to come up, check for leaks near filter, then shut it off. Check the dipstick, it might need a bit more oil once the new filter has been filled.
I'll bet it's going to hold between 4 and 6 qts. For a wager I would bet 4.75.