Bolens QT16 (model 1666) hydro/transmission fluid requirements

[Hibou]

I have a older Bolens QT16 hydrostatic garden tractor. Just got the hydro pack serviced (Sundstrand 90 series) and wondering what kind of fluid should be used/can be used.
As it stands, it looks like AW32 was used, but I'm unsure if there was a combination of dexron III and AW32 or not . Last time it was serviced a few years ago, it looks like dexron III was added. In the original manual, it states ATF type F. After going to a few stores, they said I should probably use a TDH (transmission-differential-hydraulic) fluid because the hydro pack shares the same oil as the differential. Can someone help me figure this out? Also, if uncorrect oil was used, must it be cleaned out, or can we just drain the majority and then add (10 quarts!) the right fluid?

Many thanks!

 

[MHarryE]

Are you sure about being a Series 90 because the Series 90 is a heavy duty hydro, smallest size 42 cc/rev, 6525 psi, and if I remember correctly wasn't introduced until the late 1980's, after your tractor was built. The smallest size series 90 needs over 100 horsepower to operate at its peak. Tractor Data says the Bolens has a Sundstrand series 15. Still it would probably work best with a hydraulic oil that is 12 - 80 cSt viscosity at normal operating temp which today is commonly the virtually common hydraulic fluid sold by tractor manufacturers. Using industrial hydraulic fluid AW 32 may be a little light. At one time we used AW 46 in our series 90 application before switching to Mobilfluid 424 for improved component reliability.

 

[Hibou]

It's a Sundstrand model 90, so not series 90. I reinstalled today and used ATF type F. Still curious about the diffrence with TDH fluid and if it would be ok.

 

[MHarryE]

The hydrostatic manual is very broad regarding oils that can be used - anti-wear hydraulic oil, Type "F", or hydraulic transmission oil. They give viscosity to be no less than 47 SUS at 210 degrees and no more than 6,000 SUS at the minimum expected starting temp. That suggests that the oil you would like to use is good as far as the hydro is concerned but the typical circuit in the manual shows a suction filter and Type F is thinner at low temps. It shows Type F can be used to -15 degrees and hydraulic transmission oil down to 0 degrees. Type F has a high viscosity index making it usable over a broader temperature range. Super hot or super cold Type F, but outside the Sahara or Arctic the oil you like should do well.