you should service the hydros at 75 hours and at every 400 hours thereafter. per the manual. my manual also did not say specifically to use a synthetic. I called Gravely and they said high quality synthetic oil only in the hydros. They wouldn't recommend a specific brand but my dealer said they use mobil 1 in them.
Yea, I go a little over board on the maintenance. I change oil and filter on the engine at 7 hrs instead of 10 and I installed petroleum based 10w-30 Mobile One Racing oil which has high zinc/phosphorus content for better lube. Then at today at 52 hrs. I changed the engine oil and filter again with Amsoil commercial small engine oil and serviced the hydros with new filters and Amsoil 20w-50 high zinc. In my mind the engine and hydros should be broken in at this point. Now I'll fall into the schedule of 100 hrs. or once a season on the engine and if available I'll switch from the OEM filter to an Amsoil and I'll go 400 hrs or a little early on the hydros from here on out.