Anything of reputable quality labelled Diesel 15w40 should be fine, and will be much better than was originally spec'ed when these tractors were new.
The only gotcha in modern oils is that a diesel engine made in the last 5 years may call for the latest spec oil to avoid fouling its complex emission system. That is not an issue to us with our older designs.
And - pure speculation - I wonder if a tractor used in little to no dust might tolerate longer oil change intervals now that diesel fuel has very little corrosive sulfur. I'm thinking that the anti-acid additives might not be used up so soon. Has anyone read anything on this?