I think 0w40 may be a bit thin. The heavier oils tend to have higher Shear/film strength, and the hydros likely work best with the specified high viscosity oil at operating temperature. As of now, Hydrogear actually recommends 15w50 synthetic motor oil. Let's assume they have a reason for that. A synthetic oil is stabile over time (there is virtually no thermal breakdown) and contains zero undesirable contaminants, that can not be completely refined out of dyno oils. Because there is no cat converter involved, there is no practical upper limit on anti-wear additives, such as ZDDP, so you may be able to pick something here with anti-wear additive levels on the high side. You probably do not need a detergent oil, as there are no soot or carbon deposits involved. If you want the best and do not mind the $$, I am guessing something like RedLine may not be a bad choice in the correct viscosity range. Having said that, I advise against adding some aftermarket ZDDP booster product, (blending ingredients is tricky). Given how long the oil is expected to be in there, the cost is probably not prohibitive, no matter what you pick.Have anyone tried Mobil 1 0w40 in there Hydro Gear?
Yes you can use the 15w50 it works great has high amounts of zinc many people use it, I use it all the time in my G730 , these trans axles run hot a synthetic is a must IMO.My ZTR mower has two Hydrogear 3100 Transmissions. The owners manual calls for 20W50, API SL. Can I use 15W50? I would like to use Mobile 1 they do not offer 20W50 in standard oil, only in VTwin Motorcycle oil and its API SH.