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
1 Stihl KM110 with hedge trimmer and edger
2 Stihl FS90 - Stihl HS45
Husky 435 16" - Stihl MS290 18"
Stihl BR600 - Husky BT150 - Stihl BG85
Generac 3100-PSI 2.8-GPM Pressure Washer
Jungle Jim 3TR and 2BH
82"x16' open trailer
2007 F150 FX2 5.4
Lots of Locks!
"It is not hard what we do. It's like anything else in life. Add will power, desire, and the ability to look for solutions...not excuses. Then you will succeed."