Few things:
Oil can't read a calendar so my advice is forget the "one year" stuff. If the MH is stored well in a relatively dry climate, used regularly and when used driven enough to boil off crankcase moisture, the oils is good out to several years IMO. That opinion is based on operating a variety of equipment that never reaches the miles or hours limits within one year. Over the last 15 years or so I have UOA at one year, two, years, three years and found no issues. My limit is set at four years but seldom does anything get out that far except for one truck, which is due for a change this year after four years and about 5,000 miles. Truck is an old school IDI diesel with bypass filtration.
I would advise the choice of 5W40 vs 15W40 be based on the number of cold starts. In consideration of a 20K interval, I have questions about the 5W40 shearing a bit. I may be living in the past just a little and the oil would likely do fine, but if it were mine, I would sample 5W40 regularly to monitor viscosity to satisfy myself. Maybe you can turn up some trustworthy evidence out there in posted UOAs. Back to the number of cold starts, if you have to start the MH regularly in cold weather, that's when the 5W40 is more indicated because cold starts with thick oil tend to be the ones with more wear, more filter bypass events, etc. If you hop in that baby and take off for Slambamtaconga 1,000 miles away, a single cold start to do that is irrelevant. If that's how the MH is operated, the 15W40 is the way to go IMO. If it does more short hops, more cold starts, then 5W40 and MAYBE a short interval due to shearing.
If you are set up to sample, you can conduct your own test. Use 5W40, test every 5K (or one year) or so. Then you will know ... and learn something on the way. If the 5W40 doesn't hold up, you can switch to 15W40 with confidence.
Finally, look up what oils the latest version of the N14 were authorized to use. That is always a good indicator as well.
The N14 was built to 2002 and I have seen the last N14 engine that rolled off the line. At the same time, I saw one of Cummins' earliest Model H engines of 1934, which is the direct core ancestor of the N14. Am doing a story for Diesel World magazine on this evolution. What a fantastic run and at 1999, your engine has 65 years of evolution behind it!