I don't have a Craftsman DYT 4000 but i do have a honda lawnmower with a hydrostatic trans that honda says it not serviceable. Their should be a thread in here on me servicing it.
In my research and experience with that hydrostatic drive, just about any hydrostatic drive will work with any type of oil. The most important thing is to have no air in the system and not have any air get drawn into the pump. That will cause loss of power and also the oil to heat up.
The reason why tuff-torq recommends 5w-50 is because a hydrostatic trans is a hydraulic unit, and "optimal" viscosity is 20-25 cSt at operating temperature for any type of hydraulic unit. Lower than that and you run the risk of wear, higher reduces efficiency and increases power loss. A 50w oil is rated to be 20-25 cSt at 100C operating temp, so a 5w-50 is about the best choice and will cover the unit if operated under a heavily loaded condition in high summer heat. the 5w-50 as opposed to 20w-50 just increases effiency because the viscosity will be lower under lower temps. It comes down to whether you know the operating temp the oil will see, if it's not going to get over 150-180F or the thing is used in winter then a 30 weight (0w-30) oil would be a better choice. ATF is also a very good choice and is what I use mower hydrost. drive and has been great. I think ATF would be a better choice as long as you know the fluid temp won't be getting over 80-90C, it would result in the highest pump efficiency and least pumping loss.
A hydrostatic drive has no gears, so it would not need a GL-5 rated gear oil, any oil would work and protect fine.
It would be more important for you to maintain the hydro trans with clean fluid than whether you choose engine oil over atf, 0w-30 vs 20w-50 vs 5w-50. A synthetic engine oil would generally be more efficient and a better choice than conventional oil because syn oil viscosity is lower at ambient temps.
In my research and experience with that hydrostatic drive, just about any hydrostatic drive will work with any type of oil. The most important thing is to have no air in the system and not have any air get drawn into the pump. That will cause loss of power and also the oil to heat up.
The reason why tuff-torq recommends 5w-50 is because a hydrostatic trans is a hydraulic unit, and "optimal" viscosity is 20-25 cSt at operating temperature for any type of hydraulic unit. Lower than that and you run the risk of wear, higher reduces efficiency and increases power loss. A 50w oil is rated to be 20-25 cSt at 100C operating temp, so a 5w-50 is about the best choice and will cover the unit if operated under a heavily loaded condition in high summer heat. the 5w-50 as opposed to 20w-50 just increases effiency because the viscosity will be lower under lower temps. It comes down to whether you know the operating temp the oil will see, if it's not going to get over 150-180F or the thing is used in winter then a 30 weight (0w-30) oil would be a better choice. ATF is also a very good choice and is what I use mower hydrost. drive and has been great. I think ATF would be a better choice as long as you know the fluid temp won't be getting over 80-90C, it would result in the highest pump efficiency and least pumping loss.
A hydrostatic drive has no gears, so it would not need a GL-5 rated gear oil, any oil would work and protect fine.
It would be more important for you to maintain the hydro trans with clean fluid than whether you choose engine oil over atf, 0w-30 vs 20w-50 vs 5w-50. A synthetic engine oil would generally be more efficient and a better choice than conventional oil because syn oil viscosity is lower at ambient temps.
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