Just checked my manual for my 2009 B3200 HST and It recommends 10-30 or 15-40 year round. It does give you a guide for outside temps for your choice of what oil to use. Don't know if this helps but that's what my 2009 manual says.
DevilDog
Kubota is like most care manuals so outdated. I would NEVER use a 15w-40 year round in MA.