Typically, an engine oil with a minimum rating
of 9.0 cSt (55 SUS) at 230° F (110° C) and an
API classification of SL is recommended. A
20W50 engine oil has been selected for use
by the factory and is recommended for normal
Apparently you can use any API rated SL oil whether mineral, synthetic blend, or full synthetic as there are no restrictions against using a synthetic oil.