I would use a good oil and change it at most every 5000 miles. I don't beleive a synthetic is necessary or even helpful in a street driven (is there any other kind) LS 430 since there's not really much stress on the oil at all.
Synthetics are not necessarily better than conventional oils in normal applications like the LS 430. This was not always true in the past - 20 or 30 years ago, there were some synthetics that were vastly superior to convential oils of the time. But, in 2007, conventional oils are really, really good, and oftentimes may even be better that some synthetics. Turbos are another story, and a synthetic may be better, but, again, in a relatively unstressed, extremely well-built Lexus V-8, conventional at reasonable intervals is ideal, IMHO.