Almost all SM oils from the major oil manufacturers are really good nowadays. I would not spend a lot of money for synthetic oil for an LS. I'm not just saying this lightly, as I myself, really love taking great care of my cars (including of course my Lexus) and I've done a ton of research into this subject of oil, and have concluded that "regular" 5W-30 oil (changed often enough) is probably the best way to go. I strongly feel that it's better than 25,000 miles on a really good synthetic.
I use regular (yellow bottle) Pennzoil 5W-30 in my LS400, and change it every 3 or 4 thousand miles, maybe even more if the conditions are right. If you want to be extra safe with the oil, get the Japan-made 90915-20004 oil filter.
Do you know what I mean by SM oil? If not, no big deal, just post again and I'd be glad to go into more depth on this subject.
Amsoil is very high density oil that I use for my motorcycle. As far as LS concern, I always use Castrol Syntec blend and go with factory recomended grade (5W-30). It works great for me....but its just my personal prefference. I wouldn't try to have any car run more than 5k on any type of oil or filter - syntetic or not. I think LS was engineered by very smart people...why would I want to test their knowledge?? Again....my personal opinion.