I've done on my 95 ls400 all these maintenance almost same as 91 ls400, here are something I think you should be carful to do:
1: Get your transmission fluid from your local lexus dealer, you don't need change your transmission filter,it is a metal one, you just need to clean it. But be very carfull when you want to take off the transmission drain pan,it is very hard to break it free, don't make the edge of pan uneven,also carefully clean the old sealer, I clean the filter with brake cleaner, before you install the filter back, make sure it is abslutly clean, not water,no brake cleaner at all, otherwise it may polution the transmission fluid I think. I brought a new gasket, part #35178-50010, and do as this guide you: http://www.lexls.com/tutorials/transmission/transchange.html
2:My power sterring runs good, so I don't flush my power sterring fluid, I just partial replace them with new Dox-III ATF fluid as manual suggested , each time I just pump half quart old fluid from reservoir and add new one, repeat many times until the color becomes red. If you want to flush the whole system, it is much better than I do.
3:I have switch to synthetic oil, it is no problem at all.
4:Clean MAF sensor located between main air intake and throttle body with electirc contector clean spray,do not use brake cleaner or oil cleaner! and also clean the throttle body as guide here http://www.lexls.com/tutorials/EFI/throttlebody.html.
5: I have change my ten years old fule filter last year.this was done in a local car shop, because I am little worrid about the gas dangerous.
That all what I do on my 95ls400 last year
Hope you have nice ride.