I also own a 2003 GS430 with about 93,000 miles on it - I have owned it since new. I am in a similar situation in that it does not see that much use - perhaps 8-10K miles per year.
Up until recently, I used Mobil 1 Extended Performance 5W-30. If you buy it from Wal-mart in the 5 quart jug, it's cost competitive with just about anything out there.
I recently switched to Amsoil Signature Series 5W-30. My intent is to change it once a year.
I have used Mobil 1 M1-209 filters since the car was new, and will continue to do so.
This engine is very easy on oil, as others have noticed. I do tend to agree any decent synthetic will do well in this engine given the type of service you are experiencing.