300 “type f atf” or hy-gard?

I would suggest to first talk with your local John Deere dealer Service Department for any current/updated information on fluids. If that isn't a possibility, follow the appropriate Operator's Manual recommendation. Due to the age of the equipment we are talking about and the operating clearances of the hydrostatic transmission and/or steering control valve, I always use the OM recommended fluid that is within the temperature range of the majority of the time I operate the unit.

I would also like to mention that the service interval for changing the fluid is very important, especially if the unit has been sitting for an extended period of time like a lot of the tractor "finds" talked about on this site usually are. At the risk of being banded from this forum for showing a non-John Deere tractor photo (this was all well before my 332 affliction started), see the attached photo of a 1650 Cub Cadet rear end I serviced several years ago. The tractor had probably been sitting at least 15 to 20 years when I brought it home. Note: Don't open the photo if you have a weak stomach. To change hydraulic fluid in most older Cadets, you remove the rear cover. The lumpy stuff in the photo was hydraulic fluid that had turned into basically jelly. That era of Cadets use Hy-Tran for the transmission fluid. Cadets use a variant of the same hydro that Deere hydrostats of the era use. Had I not taken the rear cover off and only changed the fluid and filter, I probably would have failed the hydro in a short time after service, or at least plugged the hydro filter. Why? Well Hy-Tran is designed to suspend a certain amount of water. Without a thorough cleaning of the rear end before fresh Hy-Tran is installed, it will "do it's job" and try to clean the system up until it is saturated with water again. BTY- it took a couple of cans of BrakeKlean, a roll of paper shop towels and my air compressor to get things cleaned up. I also took the suction line off to the hydro charge pump and cleaned it out too.
After getting the ol girl back together it ran just fine. After running it a year it, again, got new hydro fluid and filter.
So the moral to my story is: Always check with the dealer service folks on what they use when servicing your make/model/year of equipment as they should know and wouldn't use something that would result in a "come back" to the dealership if it failed/caused a problem.
Secondly, just changing the fluid and filter once may not cut it, especially if the unit has been setting for an extended period of time.