Just bought the 18/17KW model #7226 with install planned for this spring using natural gas.
The online manual on Page 27 states:
“All Generac oil kits meet minimum American Petroleum Institute (API) Service Class SJ, SL, or better. Do not use special additives.
Synthetic SAE 5W-30 for all temperature ranges.”
Was told by representative on the phone it shipped with conventional 5w30 oil in it, and that should be run for the initial 25 hours than can be switched over to synthetic. I asked if it was a break-in oil, although they couldn’t confirm that specifically.
In my temperature range for a Michigan, Mobil 1 Euro 0w40 has ran fine in my old portable Homelite gen. If wanting to follow the manual for warranty purposes a 5w30 SL rated Castrol Edge Euro, Total Quartz 9000 Energy, Mobil 1 ESP or Rotella T5 Multi Vehicle 5w30 should have near a 3.5 HT/HS. The Castrol Euro A3B4 0w30 looks really stout
Outside of warranty a 0w40 Euro or sooner depending how the 5w30 holds up after the first few early changes.