Cut this out of the "Hydro Gear manual"
Fluids
The fluids used in HYDRO-GEAR products have been
carefully selected, and only equivalent, or better products
should be substituted.
Typically, an engine oil with a minimum rating of 9.0 cSt
(55 SUS) at 230ーF (110ーC) and an API classification of SJ/
CD is recommended. A 20W-50 engine oil has been
selected for use by the factory and is recommended for
normal operating temperatures. For colder climate operation
(see chart), a change to a 10w-40 engine oil may be
necessary. Another alternative that will provide excellent
all climate performance and extended time between oil
changes is a 15W-50 synthetic engine oil.