[M.D. Holloway]

Thats easy - For your Lexus - use Toyota Type T IV ATF fluids.

Most domestic history: Tranny fluids can be divied into two major groups - those that contain friction modifiers and those that do not. Friction mods are chemicals that make the fluid more slippery under certain conditions. Until the mid 70's, Ford produced trannys with small frictional surfaces. The bands and clutches had to be applied quickly with high pressure to prevent over heating and wear caused by excessive slippage - this requires a non-friction modified fluid with a higher static of coeefficient of friction than a dynamic coeeficient of friction - in other words the frictional forces between the clutches and bands are greater when there is no reletive motion between the components then when they are slidiong against one another before lock-up AKA the hard shift.
GM trannys require a more slippery fluid to permirt a smooth shift without any shudder. When the clutch or band locks ups, the friction mods react to decrease the coefficient of dynamic friction - this allows the use of low clutch and band pressures and more plates in the clutch packs to prevent excessive slippage and heat build-up.

Using the wrong fluid in either tranny style will result in poor shift quality and short component life.

For Ford model years, use the following spec's fluids:
1949-1960: Type A
1960-1981: Type F
1977-1981: Type CJ
1982-1987: Type H
1988-1992: Mercon
1992-present: Mercon Revised
1997-present: Mercon V

For GM model years, use the following spec's fluids:
1949-1966: Type A
1967-1973: Dexron
1974-1991: Dexron II/IID
1992-1994: Dexron IIE
1995-present: Dexron III

For Chrysler model years, use the following spec's fluids:
Pre 1988: Dexron IID
1988-1997: ATF Plus, ATF +2
1998 - Present: ATF+3
1999 - Present: ATF +4