Air cooled engines run hot. Heat is a conventional oil killer. We run amsoil in everything and change it twice a year. Amsoil has a very high NOAK volatility rating which means it doesn't break down and burn off as quick as other oils.
2013 Gravely 460Z 29EFI
2011 Gravely 36GR
2008 Gravely 160Z
2004 Gravely 34Z
1998 Lazer Z HP 48 (old faithful)