The general consensus is that synthetic/semi synthetic oils are fine for rotaries, AS LONG AS you disable the metering and either run a 2-stroke additive or an external mineral oiling setup.
I'm sure people have run straight synthetics but I'd be sketchy myself.
have u actually owned a 13b before?
i have NORMAL metering and NO ADDATIVES.
and i use SEMI SYNTHETIC halvoline oil as recommended my mildrens my rotor workshop (the most respected one in adelaide) on all their cars.