You don't say what MY you have. I have an 06 & the OM states premium, but regular can be used if premium is not available.
When I was doing research prior to buying the vehicle, using the Lexus.com website, the vehicle comparison feature was powered by edmunds.com & it stated that the vehicle (GX) used regular unleaded.
Btw, the sticker inside the fuel door of my 06 states Premium.