Heavy diesel oil

[Hoosier Hay Man]

I have a 1968 4020 JD and run rotella 15w-40 in it. Is there a heavier oil i can get away with? The oil seems a little thin when changing. The motor seems strong, smokes until warm.

I have a1974 JD 4430 that the owners manual recommends 30wt diesel oil in the summer and 10w30 winter. I am guessing 15w40wasn't common then. We have always ran 15w40 year around.