Your local tractor dealer is apparently not familiar with Case garden tractors and his recommendation would have landed you in trouble.
Ken Combs is correct in telling you to use only motor oil in the hydraulic system of your 600 series Case. And this advice holds true for all the GT's with Hy-Drive produced by Colt, Case or Ingersoll.
While oils blended for diesel engines are an excellent choice, I prefer any high quality 20W50 or straight 30W for the best performance from these tractors. If you are in doubt about what's in your tractor for oil right now, then drain all hydraulic cylinders along with the reservoir before installing the new oil.