Lexus does not recommend "special" oil, nor does it recommend synthetic oil. It recommends 0W20 and 5W20 petroleum based oil. Check your owner's manual. This quality brand name petroleum based oil can be purchased at most automotive stores for $3-$4 per quart. Your dealership is trying to make an excessive profit and hoping you will cooperate.
It recommends 0W20, or 5W20 if you can't find 0W20. If you use 0W20, it pretty much will be synthetic. You can get 5W20 dino oil, but have not seen nor can I find 0W20 dino oil. Which grade are you using? If you are using 0W20 dino, what brand are you using, because I can't find 0W20 dino oil from Mobil, Valvoline, Pennzoil, or Havoline.