Q: Will a u140e trans from a 99 Lexus es300 interchange with a 99 Lexus RX 300? Fwd?

Although the transmission type is the same (U140-E), the two vehicles have different Tag ID numbers on the transmissions and thus the two referenced transmission are NOT listed in the catalog as "interchangeable". Different Tag ID’s reflect differences in electrical controls and possibly differently calibrated valve bodies. At any rate, to answer your question the two transmissions are absolutely not compatible and to determine what retrofits "might" make them compatible would require quite a bit of research (it is possible to arrive at a determination, but you would have to check quite a few details and be very careful as these components are all computer controlled; matches have to be exact). If you are looking for an exact interchange, simply go to Car-Part.com and you will find an exact match for your car (there are many listed). Transmission swaps on modern computer controlled vehicles must be exact matches because if you attempt to use a transmission that is not cataloged as a direct cross reference, you will find that the engine and transmission computers will be incompatible. Always check the transmission Tag ID code. If the Tag ID of the proposed transmission does not exactly match the existing application, it will not work. Even if you can get a "different" transmission to physically fit, it will not operate properly and the PCM will spit out error codes. If you are interested in a transmission replacement (OEM to OEM), YourMechanic professionals in certain locales can assist with that. Please simply inquire based on your locale. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.