I concur with Jim on each of his points, including the fact that I, too, use Rotella 10w30 year round in my Kubota.
All the compact utility tractors from Deere use Yanmar engines, if I'm not mistaken. They have rightfully earned a solid reputation for reliability.
"Best" oil? That depends upon your point of view.
"Best Buy" oil? A common dino HDEO in either 10w30 or 15w40, used year round. Cannot beat the performance/dollar ratio. If your FIL is close in proximity (MN), I'd lean towards the 10w30. If he's towards the south, then the 15w40. Pick any decent brand name you like; it won't matter.
If people knew how equipment is treated (abused) in third world countries, they'd not worry about little things such as this. I understand everyone wants the "best" for their new little gem, but the reality is that the Yanmar could practically run on sewing machine oil and still labor on ... (I exaggerate, but not by much.)