The greenhouse Onan is a 10 KW generator.
From the plate on the generator:
This model was built between 1954 and 1966
For those who have a similar 10CW Onan here is the cross reference for the oil and air filters for this engine:
Carquest 85010
NAPA 1010 (approx $10)
Fram C3P
10-30W Detergent oil\ capacity: 6 quarts (or 7 quarts if changing oil filter)
AC Delco A142C
Wix Filter 42371
Fram CA73
Champion K-15J which has been replaced by Champion D14 use .025 gap