Welcome to CL.
The 1Gen IS was extremely well built (I owned one) and was one of the most reliable products Lexus ever built. But it was not without its weak points. The 3.0L in-line six guzzled premium gas like a V8. The automatic transmission sometimes hiccuped or flared now and then on 2nd-3rd upshifts, even after a factory-approved ECU reprogramming. Rolling acceleration from slow speeds sometimes exposed hesitation from quirks in the throttle-by-wire system. And the shift-lever itself, though with a nice snappy-positive action and the chrome ball, also had a few quirks in how the designers designed the shift-gate itself. Other than that, though, it was a superbly-built car, although by now the latest ones (2005) are now some 11 years old....so expect wear. You'll probably have to replace some things.
If you want to avoid those automatic-transmission issues, of course, you can look for a 5-speed manual version, but much fewer of them were built than automatics.
As far as oil and spark plugs go, it is usually best to use what is recommended in the Owners' Manual unless circumstances dictate otherwise. One exception would be if your car has enough wear on the engine to be significantly affecting oil consumption. Then, sometimes, high-mileage oil can help, as it is a little thicker in viscosity, with different additives, and won't leak out through openings or blow by worn rings/valves/bearings as fast. But with thicker oil, starting at low temperatures could be more difficult for the battery and starter-motor (more drag), and it could also affect gas mileage, depending on the wear-stage of the engine and outside temperatures.