Ed, 

I was thinking the same thing about the anti-foaming agents. The basic component of the two oils is the same, it's the additives that make each unique. I'd drain the motor oil and put a good 10W hydraulic, or even a universal hydraulic oil in its place.

One other thing. I have a service bulletin for the Super 55 that pertains to putting a longer return tube inside the reservoir, to prevent foaming, especially with an external valve. It would return the oil near the bottom of the reservoir, reducing the amount of air that can mix in. I would think this would have been incorporated into your 550 by the time it was made, but maybe someone had it apart and didn't see the importance of the return tube and didn't re-install it. To check this out would require the removal of the three point/hydraulic cover. I'd try changing the fluid to a true hydraulic oil first.