Think about this scenario (I'm not saying it's true, I wouldn't know).
1. Maybe it wants EP90 in the hydro pump, but you've been using SAE30.
2. It's never made any funny noises, it just won't drive when the hydro oil warms up.
3. Just maybe, the reason it doesn't drive is that the hot SAE30 is so thin, the hydro pump can't pump it up to pressure - it leaks back through the pump and output pressure is very low.
4. If that happened to all be true, it may be that just by putting the right oil in it, you will find you have full drive capability, and not much damage has been done to anything.
You need to be very sure what oil that hydro unit is supposed to use. I wouldn't want to put EP90 in one made for SAE30, or you might wreck it. But if you put SAE30 in one made for EP90, I wouldn't expect much output pressure once the oil heated up, if it had any wear on the pump.
