If you are talking about using it as motor oil, don't even think about it!!! It doesn't have the right additives and will cause lots of wear.
If you are thinking about adding it to your fuel, well, I'm not too sure about that either. I use a lot of used motor oil in my diesel fuel. I have done it for years without a problem. However, I have not had good experience with hydraulic oil in the fuel. Any of it I have tried to burn just seemed to gum things up and I had to run a lot of injector cleaner to get things running right again.
You might try adding a small amount to your fuel... a quart or two to a full tank of diesel... and see what happens. If it burns good, go for it. If not, forget it.
Steve