I would guess that it is a 10W40 but that is only a guess. The dealer might be able to tell you but even they might not be entirely sure.
How much is a little low and has it gone down at all since you took possession of the tractor. If it is only slightly below the full mark and hasn't moved then I wouldn't be the least bit concerned, I usually prefer to run engines slightly below the "full" mark. If the oil level has gone down noticeably in 20 hours that is more of a concern. My previous tractor and the current one were both bought new by me and I did not have to add any oil, in fact could not tell that any oil had been consumed, between oil changes including the first one.
If the tractor is using oil I would speak with the dealer about that to get it on record and then monitor how much oil is being used in case it becomes a warranty issue.