2009 CubCadet GT2550 Hydrostatic transmission

Hello,
I'm new to the forum. Just went to pick up my Cub GT2550 mower from my Cub Cadet dealer (where I purchased it) for a repair. The hydrostatic transmission(cast iron axle) had a leak. I asked them to double check the fluid and then the repairman proceeded to use Castrol 20W50 motor oil instead of the Cub Cadet hydrostatic oil #737-3120. I asked him why he did use the Hydrostatic oil #737-3120 and he said was told 20W50motor oil was fine. Is this truly OK. I have only ever run the CC HydroStatic oil #737-3120 in it. Before I start using it, I need to be sure and of course the factory support line is closed, today.

Thanks for any assistance!

Send a query to Cub Cadet direct.
That dealer will end up getting his fingers smacked.
Most hydros I have worked on run on engine oil, not hydraulic oil.
However just about all of them run fully synthetic engine oil, not plain mineral oil.

I don't think there is a direct crossover from the special hydro fluid Cub Cadet uses to an equivalent but Hydrogear uses 20w50 oil and some of the Snapper/Simplicity mowers use 10w30 with the tufftorq transmission. Tufftorq normally uses a 5w50 synthetic, so I don't think the 20w50 will hurt anything, just not what the mower manufacturer uses.

I have seen other mowers call for 15w40, 10w40, 20w20 hydraulic oil,

Best thing to do is to check the manual for the hydrostatic. If it is in fact a Hydro-Gear, them they can provide you with the manual or at least some insight into the fluid requirements. I personally would feel better if it had the same fluid that it shipped with new. In my case I have an Exam that the hydro reservoir has a sticker that says to only use Mobil 1 synthetic 15w-50 oil, as well as the operators manual stating the exact same, yet 12 years later I cannot use that oil because it simply isn't formulated the same. At the end of the day, there was a suitable replacement for the oil that works just fine. So who knows? Good luck resolving your issue.