15-40 or even 10-30 in winter with light load.
IH started allowing 15-40 in the old engines that originally were only #10 or #30 motor oil, starting in 1983, stating that you should not run multiweight more than 100 hrs.
Use hytran in everything else and remember that the final drive / bull gear housings do not share hytran with the main transaxle. Each final drive housing has a fill / check plug on the rear side. Fill with hytran up to the plug hole.