Hydrostatic transmission, often abbreviated to "hydro drive," is a propulsion method used since the early 1970s, mostly on lawn tractors and mowers. Kubota, Husqvarna and HydroGear manufacture most hydrostatic transmissions used in the U.S.; their products are installed by Sears in its Craftsman range, and by John Deere. The transmissions work using hydraulic oil.
Because of the range of environments in which lawn care equipment is used, it is not possible to give specific advice on oil type for all applications. John Deere notes on its website that the air temperature range in which hydrostatic machinery is used must be taken into account when determining the viscosity of replacement oil. The company states that "Operating outside of the recommended oil air temperature range may cause premature hydrostatic transmission failure." For this reason it is imperative to choose the correct oil not just for your machine, but also for your location. Between minus 4 degrees and 122 degrees Fahrenheit, John Deere recommends SAE 20W-50 be used on its products. The company also recommends that a full drain of the drive take place before using new oil, so that no mixing takes place. The company also strongly advises against bio-based oils and against red "Type F" automatic transmission fluid.
When to Change the Oil
Poor performance and drive-train hesitation after cold-starting are seldom indications that the oil in a hydrostatic transmission needs to be changed. The most common indicator is performance fade as the machine heats up; if the machine is slow to respond in both forward and reverse, or if the machine decelerates on upward slopes, the oil may require changing.
What Happens to Old Oil
Older oil loses its anti-foam capacity as the additive package decays. One result of this decomposition is that both air and atmospheric moisture becomes mixed in with the oil, a process known as entraining. Entrained oil does not possess the correct hydraulic properties, so the drive system will not function optimally. At best the entrained oil will result in the machine slowing noticeably on even mild slopes; at worst, the machine will not self-propel at all.
Hints for Changing the Oil
It is imperative to entirely drain the old oil. Park the machine at the draining angle advised by the manufacturer — this is usually dead level — and remove the drain plug. Then leave the plug out for several days before replacing it and installing the new oil. If the system has a purge function, follow the manufacturer's directions to purge. Ensure the reservoir is filled to its optimum mark at filling, and check again at five-minute intervals through the first use, topping up as necessary to ensure the optimum level is maintained.