RE: Onan / Cummins - LP Fuel

From the Manual. HD-5 is a standard LP mixture:

LP Generators — Use clean, HD-5 grade liquid propane in a mixture of at least 90% propane. Propane fuels other than HD-5 can contain more than 2.5 percent butane which can result in poor fuel vaporization and poor engine starting in outside temperatures below 32°F.