While not an absolute, I would use all synthetic lube fuels in the YS to keep from gumming up the works.  Otherwise, I always have some Castor in the mix.  And I would bring Omega up to 20% total lube by adding some Castor to the mix.  My old Saito and Magnum engine manuals both recommend the use of 20% lube.  Magnum as previously mentioned also recommends some Castor.  Maybe someday we will have those superior synthetic oils used in Europe but until then I always add Castor to the mix except for the YS.