I can't tell you whether it safe is to use 10w40 in your hydro system or not but I can tell you what I do. I have a Dixie Chopper. When this mower was purchased it was recommended that you use Dixie Chopper's proprietary oil VTC. Then Dixie decided that they like Lucas 15/40 High TBN Truck oil. Now Dixie appears to be having a spat of some sort with Lucas so who knows what the oil of choice will be tomorrow. That said, I use Mobil 1 15w50 in my Dixie and a whole lot of other people are using it too. The issue of using the wrong weight oil comes into play when the hydro system is cold. The oil doesn't flow as well when it's cold. They size the hydro filters to match the oil. As long as you stick to a synthetic that is right around the same range as the original you should be fine.