Personally I think your dealer is giving you BS. First the car calls for 5w20 or 0w20 and since I have never seen 0w20 in anything but a synthetic I have to beleive Toyota knows that and honestly what woudl their reasoning be to NOT use a synthetic, it hold up better to abuse and high temperatures and from a manufacturer that had sludge problems you would think they would welcome synthetic. I suspect your dealer is very misinformed or an idiot or possibly both