My dealer explained to me the difference. As stated by mfreund, it's all about temperature. The low viscosity is for colder climates.
I asked my dealer, because I was replacing the hydraulic fluid in an OLD late 1950's Yale forklift. The manual said to use "turbine" quality oil and listed a viscosity index number. I used regular Hy-guard (since I have lots of JD tractors and buy it by the 55 gallon drum), but when cold, below 40F, I notice it takes some time for the pump to prime (pump is HIGHER then the tank).
My suggestion to using Hygard vs low vis in a hydrostatic tranny, let it warm up. If you HAD standard hyguard and you said low vis is quieter, it's (just a guess) the pressure bypass. I have a few old Jacobsen law tractors that have a hydro-stat trans and it squeals when cold (it has different oils recommended too....but one was similar to JD Hyguard-aka hydraulic/transmission oil). If you start it up for winter/cold weather use, start it up, let it idle 5 min....then bump up the throttle a little. If you have a front end loader, cycle the loader up down 3-5 times after warming. Just don't start the thing and in less then 1 min, stomp on the go peddle. You could, but letting it warm up is better.... I do that w/ my JD's when I need to use the hydraulic systems.