I found this thread & post by "ddbtdd", so I guess I'll go with Mobil 1 full synthetic, as the Great Dane book says.
http://www.lawnmowerforum.com/grave...aulic-oil-type.html?highlight=hydrostatic+oil
ddbtdd says:
"I recently changed the oil in my hydros but before I did I called Hydro-gear and asked what they recommend.
The man I spoke to said all Hydro gears ship out with MOBIL conventional oil.
It was alright to use synthetic but not necessary.
So then I called Snapper Pro. They suggested synthetic but said it was not necessary.
So what did I use? Valvoline Full Synthetic 20w50 just because I can!"
I also found:
All these filters cross reference to each other and are interchangeable.
If your machine uses one of these, you might want to save this.
AC Delco...........PF1685
Bad Boy............063-8014-00
Baldwin.............BT346
Fram.................P11588
Gravely.............GDA-10137
Great Dane........GDA10137
John Deere........M131053
NAPA................1200
NAPA................1519
Scag.................48462-01
Snapper............2-7164
Snapper............7027164
Wix..................51200
Wix..................51519
All have these specs:
Thread Size: 1 " -12
OD: 3 3/4 "
Height: 3 7/8 "
Micron: 10