Pick the transaxle under your mower and you get what Hydro-Gear’s own service manual says for that family: the oil, the interval, and — for two of these four — the fact that the unit is sealed and was never meant to be changed at all.
ZT-2800 / ZT-3100 / ZT-3200
20W50 engine oilAn initial oil and filter change at 75–100 hours, then every 400 hours thereafter. Hydro-Gear does not publish one figure for this family: “Correct oil volume is determined by characters 7 and 13 of the model number.” Read those two characters off your own unit rather than trusting any number on the internet — including ours, which is why there is not one here.
- Fluid
- 20W50 engine oil
- Capacity
- not published as one figure
- Service
- An initial oil and filter change at 75–100 hours, then every 400 hours thereafter.
Your mower’s own manual outranks this page: the machine’s maker sometimes specifies a different fluid or interval for the same transaxle. And whatever you drain, the drive has to be purged afterwards — air in a closed loop feels exactly like a worn-out pump, and it is the reason most “my hydro is weak after an oil change” posts exist.
Hydro-Gear manuals checked 2026-09-24.
Quick answer: Hydro-Gear specifies 20W50 engine oil across these transaxles — “an engine oil with a minimum rating of 9.0 cSt (55 SUS) at 230°F and an API classification of SL”, with 20W50 chosen at the factory. The ZT-2800 / ZT-3100 / ZT-3200 wants an oil and filter change at 75–100 hours, then every 400; the ZT-4400 at 100 hours, then every 400 or yearly. The EZT (ZT-2100 / ZT-2200) and the T2 / T2-HP / T3 HD are different: Hydro-Gear says each “is factory filled, sealed and does not require oil maintenance”.
How to use it
Find the model number on the transaxle itself — it is on the housing, either on a label or cast into the metal, and it is not the same as the mower’s model number. Pick that family above and you get the fluid, the capacity where Hydro-Gear publishes one, and the interval in their words. Put your hour-meter reading in and it tells you where you are against that interval.
Sealed is an answer, not a gap
Two of the four families here are not on a service schedule at all. Hydro-Gear’s manuals for the EZT and for the T2/T3 both carry the same line — “This transaxle is factory filled, sealed and does not require oil maintenance” — and that is worth knowing before you buy two quarts of 20W50 and start looking for a drain plug that does not exist. The EZT manual does also print a procedure for “models with a fluid drain port”, so some units in that family can be drained; look for the port first.
Sealed does not mean immortal. It means there is no scheduled change. If the oil is dark and foaming, or the machine is slow up a slope on one side, that is a repair, not maintenance.
What each family takes
| Transaxle | Fluid | Service |
|---|---|---|
| ZT-2800 / ZT-3100 / ZT-3200 | 20W50 engine oil | 75–100 hours, then every 400 |
| ZT-4400 | 20W50 motor oil | 100 hours, then every 400 or yearly |
| ZT-2100 / ZT-2200 (EZT) | 20W50 engine oil | sealed — none scheduled |
| T2 / T2-HP / T3 HD | 20W50 motor oil | sealed — none scheduled |
The capacity is the part people most want and the part that is least publishable as one number. For the ZT-2800 family Hydro-Gear says outright that the “correct oil volume is determined by characters 7 and 13 of the model number” — so there is no single figure, and this page does not print one. For the EZT it publishes 54 fl oz (ZT-2100) and 56 fl oz (ZT-2200); for the T2/T3, ranges of about 65–70 fl oz per transaxle, because the fill is to a level rather than to a bottle count.
Why engine oil in a transmission?
It reads like a mistake and it is not. A hydrostatic drive runs its pump and motor on a film of oil at temperatures that reach 230°F, and Hydro-Gear’s spec is written around that: at least 9.0 cSt at 230°F, API SL. A 20W50 engine oil meets it. A thin hydraulic fluid does not, whatever the colour of the bottle suggests. If you substitute something, match the spec rather than the label.
Purging: the step that gets skipped
Every hydrostatic system that has been opened has air in it, and air in a closed loop behaves exactly like a worn pump — weak drive, one side lagging, creep when the levers are neutral. That is why the manuals carry a purging procedure. Wheels off the ground, engine running, bypass open, work the controls slowly forward and back until the movement is smooth and the noise settles. Skipping it is the reason for most “my hydro got worse after I changed the oil” threads.
When the numbers disagree
Hydro-Gear builds the transaxle; your mower’s maker builds the machine around it and sometimes publishes its own fluid or interval for the same unit. Where they differ, follow the machine’s manual — it knows what else is plumbed into that loop. If you have lost it, the model number off the transaxle will get you Hydro-Gear’s own manual free from their site, which is where every figure on this page came from.