Toro recommend using a premium hydraulic oil; such as Mobil DT 26 in Eatons trannys
Eatons are worth the extra cost & effort of using a genuine hydraulic oil
They also specify it must be filtered to 5 µm
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If you use engine oil then full synthetic is worth the extra price but note the minimum viscosity is SAE 20
Hydraulic oils like gearbox oils are measured on a different scale to engine oils
I think I used Shell agtrans 80 in the boxes I do because I have 50 litres of it
But I checked with Shell first
And Eatons get purged differently
They should self purge the air
The recommendation is to raise the wheel off the ground
Start engine & run at the slowest possible speed for 5 minutes
Then work the F-R pedal a few times
Turn the engine off & leave it 15 to 30 minutes
Adjust the oil level if needed
Check for drive
If it is spongy repeat the above .