Hi Jiogmaster
My only experience / learning is from picking up on the various threads on this Forum.
I'm not advising against, just commenting on what I have seen perhaps being the concensus relating to the LS400 ................ not the LS430.
ManicMark suggests above, going to Lexus for the Gear Box full 20 litre flush and refill at £140 ......... not a DIY prospect maybe on this specifically.
Memory says, I think, that partial drain and fill might be a total waste of time on the LS400............ untouched it's an incredibly smooth gearbox.
In 10 years of driving 3 X LS400s:, Mk1 and Mk2 and now the Mk3 and covering perhaps 150,000 miles, I have never had to have a gearbox even topped-up ! not even ( knowingly ) on scheduled servicing.
Malc