You can use 20W50 or 15W40 in the hydraulic system and in the trans-axle.
In warm weather, the engine should have ONLY straight 30 weight oil. In the winter, 10W30 in the Kohler is OK but keep a close eye on the level because the engine will use multigrade oils faster than straight grades and the oil pan on Case spec'd Kohlers is smaller than on other tractor brands.
Therefore, oil level is crucial if you want the engine to last. ♠