I have a '99 RX300 with ~88K miles and had it for almost 2 years and changing my own oil. I have been using penzoil high mileage and working great. However, my recent 2-oil changes, a good friend of mine who has a history with racing and has his own mechanic shop to consider to switch to Royal Purple Synthetic. Let me tell you, night and day. After putting that in my vehicle, it felt like it gave it a steroid shot. Idles smoothly, increased mileage, lowered temp, and appears to make the engine not struggle during hard acceleration.
Skeptical at first, but a believer now. A good grade oil like a Penzoil or this Royal Purple, will make the RX fun for quite a while...Good luck!