My JD is a 1990, 625 Series w/ the hydrostatic transmission that I bought used with about 500 hours on it ....
I'd down here in Northern Alabama and I have been running the Amsoil 10w-30 synthetic for the past three seasons with excellent results. This engine has a spin on, full flow oil filter and my oil is barely dark after 60-75 hours of use.
I also plan to change over the transmission to the Amsoil 5w-30, hydraulic/transmission fluid, which exceeds all the current JD specs. Just one of those things I haven't gotten around to yet....
Bottom line is that any high quality, 10w-30 synthetic will work well here. I'd recommend the GC/0w-30 or perhaps the Redline 10w-30. You could also try the new Mobil 1, 10w-30 Extended Performance formula.
One thing I would highly recommend is that you run 91-93 octane fuel in hot weather. My engine vapor locks badly on 87 octane and fuel w/ 10% ethanol is to be avoided all together.
Tooslick