If I read Bruce's post correctly, the proper oil for that hydrostatic unit is EP90, and you are using straight SAE30. You also said that the problem does not arise until after warm-up of the unit. I may have misunderstood both of you, but if my summary above is correct, it sounds like you may have been boiling the light oil you have incorrectly put in the hydrostatic unit and it has cavitated extensively. There may be pretty severe damage if that is what has happened. Of course severe under-filling or an air-lock is another possibility.
Usually cavitation is accompanied by some distinctive noises, in my experience typically sounding like a loose ball from a ball-bearing getting smashed to pieces by going through some gears. That is followed by loss of drive, sometimes due to almost complete destruction of the hydraulic pump. Well, that is how it shaped up when it happened to my front-loader backhoe anyway. Boy that was expensive.
