I have a 2002 RX300. Bought it with 29k miles in Nov05, and had dealer examine to verify no sludge. Since then, Mobil-1 only, every 7k miles. I drive 40k hwy miles/yr. At 140k miles now, I discovered (sudden brake caused oil light to come on) that I am losing 1qt every 2-3k miles. Absolutely no leaks. Had dealer ck for sludge. Engine clean. (not checked plugs) They "researched" Lexus info, and declared that my usage is within acceptable range. They reasoned that on the highway there could be more usage than around town. Car rides fine. I figure a qt every 2-3k is cheaper than any mechanical repair. Performance is good.
I did purchase a 2001 RX before this one, which was sludged. Broker took it back, and found this 2002 (proud he stood behind the sale!!). The sludged '01 consumed oil and also had "engine ping" when accelerating up a hill.
I'd monitor yours and obtain an accurate measure of oil usage.