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.
2011 Gravely 252 26HP
2011 Gravely Pro Walk 36 Hydro 16HP
2 Stihl KM110's with trimmer and edger
Stihl HS45 - Stihl FS90
Husky 435 16" - Stihl MS290 18"
Stihl BR600 - Husky BT150 - Stihl BG85
Deere 3800psi/4GPM pressure washer
Jungle Jim 3TR and 2BH
82"x16' open trailer
1998 F150 245k
A Whole Bunch of Locks
"It is not hard what we do like anything else in life, added will power, desire and the ability to look for solutions not excuses then you will succeed."