Coperion MEGAcompounder Gearbox foaming Problem

[davidtb]

We got a ZSK-70 Megacompounder loaded with Castrol 1715-320 semi-synthetic, it's at 120°F running 50PSI spray gearbox. We get foaming from the top at start up, and from the sample port 1" from the bottom of the tank, theres entrainment. Some think a switch to a full synthetic oil will do the trick, I'm not so sure. We did an ultra sound for leaks, and found some, they got permatex'ed, there's no line restrictions or resistance, no turbulance or shock wave, no external forces. The problem I have nagging me is the temp of the oil, and a feeling of unforeseen cavitation, for the life of me, I can't figure out where. Some feel that the entrainment is do'able since it's a spray box anyways, but I want them to tell me that after the 2nd or 3rd oil change. Any help on the "unforseen" is something, really.
Thanks
db.